Two cards are very common to a lot of Saffi builds, these being Boonweaver Giant who forms a combo with Pattern of Rebirth, and Protean Hulk who can fetch a handful of combo creatures in one hit. This list however runs neither of these. There has been a lot of discussion regarding these two cards and their deliberate exclusion which is covered individually below:
Why no Boonweaver Giant?
For the uninitiated, the basic premise behind behind the boonweaver combo is to either play Boonweaver Giant, fetching Pattern of Rebirth, or by playing Pattern of Rebirth on a disposable creature and sacrificing them to get Boonweaver Giant which in turn gets Pattern back from the graveyard. From here and with the aid of a repeatable sac outlet, the following chain is:
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Sacrifice Boonweaver Giant enchanted with Pattern of Rebirth fetching Karmic Guide
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Karmic Guide returns Boonweaver Giant which in turn returns Pattern of Rebirth
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Sacrifice Boonweaver Giant enchanted with Pattern of Rebirth fetching Felidar Guardian
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Felidar Guardian flickers Karmic Guide, repeating step 2
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Sacrifice Boonweaver Giant enchanted with Pattern of Rebirth fetching Reveillark
From here you can form the loop of Reveillark + Karmic Guide + Felidar Guardian which can generate infinite mana by flickering a land, and then can cast and use Saffi Eriksdotter + Reveillark + Karmic Guide + Boonweaver Giant + Pattern of Rebirth to tutor any creature in the deck (such as an Elvish Mystic) and assemble a win condition.
While this combo seems good at first, there are a few issues with it. The first and the most glaring issue is that you need an infinite sacrifice outlet to kick off the loop. As this deck is not designed to be crammed full of sacrifice outlets which don't generate value in their own right, there are really only 5 loopable outlets to begin with. Two of these net infinite mana (which is the best selling point for the inclusion of the combo), one of these can draw the deck and the others are outright win conditions.
The second issue is that since Saffi is our commander, it is assumed that we will generally have access to her. At this point, Pattern of Rebirth and Boonweaver Giant could be any creature tutor which would in turn fetch any Saffi loop. And since the deck runs such a high density of draw/tutor effects, assembling Phyrexian Altar/Ashnod's Altar loops without a way of turning it into a win conditon where a Boonweaver loop could is quite reduced.
As a third point, Pattern of Rebirth as a creature tutor outside of combo is not an efficient tutor. The major problem with Pattern is that you need either a creature and a sacrifice outlet or a self sacrificing creature to make it work. Yes, Saffi fits that bill, however having to use Saffi in this context to not fetch a combo creature is a bit of a waste of Saffi. As well, you need some form of boardstate to make it work, unlike cards like Green Sun's Zenith, Finale of Devastation or Eladamri's Call.
Why no Protean Hulk?
Since its unbanning, Protean Hulk has been been the centerpiece of many graveyard combo decks. These typically use Flash (haha, banned now) or Necromancy to deploy Hulk which in turn gets some sort of creature based combo. Again, for the uninitiated, the line is:
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Sacrifice Protean Hulk fetching Karmic Guide
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Karmic Guide returns Protean Hulk
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Sacrifice Protean Hulk fetching Felidar Guardian
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Felidar Guardian flickers Karmic Guide, repeating step 2
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Sacrifice Protean Hulk fetching Reveillark
From here you can form the loop of Reveillark + Karmic Guide + Felidar Guardian which can generate infinite mana by flickering a land (as above), and then can cast and use Saffi Eriksdotter + Reveillark + Karmic Guide + Protean Hulk to tutor any 6 CMC or less creature in the deck (i.e. all of them) and assemble a win condition.
Unlike Boonweaver Giant however, Protean Hulk is able to assemble a combo from a single use sacrifice outlet with the assistance of Saffi Eriksdotter.
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Sacrifice Saffi Eriksdotter, targetting Protean Hulk
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Sacrifice Protean Hulk to a one shot sacrifice effect (eg. High Market), fetching Academy Rector and Starved Rusalka and returning Protean Hulk
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Sacrifice Academy Rector to Starved Rusalka fetching Greater Good
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Sacrifice Protean Hulk to Greater Good, stacking triggers so that you fetch before draw/discarding
From here, depending on the available mana (and the mana rocks still in the deck), use the Protean Hulk tutor to fetch either Sun Titan or Karmic Guide, both of which return Saffi, and then follow the above desired Greater Good line.
With the inclusion of some additional cards, there is also a line which doesn't rely on Greater Good:
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Sacrifice Saffi Eriksdotter, targetting Protean Hulk
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Sacrifice Protean Hulk to a one shot sacrifice effect (eg. High Market), fetching Stoneforge Mystic who in turn gets Demonmail Hauberk and returning Protean Hulk
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With the additional 4 CMC worth of creature tutors and to help generate the mana to cast Demonmail Hauberk, you can fetch Hidden Herbalists, Lotus Cobra and Dryad Arbor
From here, you can use the equip cost of Demonmail Hauberk as your sacrifice outlet and then follow the standard Protean Hulk line listed above.
While the above line does bring additional utility to the deck through a creature tutorable sacrifice outlet, to make the combo work requires a lot of support cards not currently in the deck, or greatly increases the cost of the combo (by the cost of Hauberk).
Because the deck does not have access to black for renamiation effects like Necromancy, either Pattern of Rebirth or Natural Order are required if you are wanting to shortcut having to hardcast Hulk. The shortcomings of Pattern of Rebirth are discussed above and unfortunately Natural Order suffers in a simliar way. While Natural Order has the benefit of not needing either a self-sacrificable creature or sacrifice outlet, it only has the ability to tutor a green creature, making it much more restrictive outside of fetching Protean Hulk.
As mentioned above, while Protean Hulk does have the ability to initiate a combo without a repeatable sacrifice outlet (unlike Boonweaver Giant), it is still a very mana intensive combo. With Pattern of Rebirth it might seem cheaper at first, but unless your sacrifice outlet is Starved Rusalka then you run into the issue of needing both a sacrifice outlet for the enchanted creature and Hulk, or need to enchant a creature like Saffi herself, which in turn causes issues in that you need Saffi in play to target Protean Hulk. While it is still possible to make this work (via fetching Karmic Guide + Starved Rusalka into Reveillark before fetching Greater Good), this then doesn't end up reducing the mana cost significantly. Additionally, if you have fetched Pattern of Rebirth with Academy Rector, you can no longer use it to fetch Greater Good, effectively Catch-22ing yourself.
The biggest selling point for Protean Hulk is the interaction with Natural Order, allowing you to turn a mana dork into Hulk. From here, you can follow either of the above lines, depending on if you have a repeatable sacrifice outlet or not. While this does lend itself to the inclusion of Protean Hulk, it is not enough of a selling point for its inclusion.
It is worth noting that between Protean Hulk and Boonweaver Giant, Hulk is generally the better choice as both lines work next to identically with Pattern of Rebirth (assuming a sacrifice outlet) however Protean Hulk does provide some additional lines which are covered above. Hulk also inteacts with Natural Order and Green Sun's Zenith whereas Boonweaver Giant does not. If however you do look to include some / all of the enchantment package in your particular build (see below), then Boonweaver Giant might provide more value for you due to its interaction with Gift of Immortality.
So why Woodland Bellower?
With the introduction of the Yisan deployment line into the deck, we saw a new high CMC creature added to the fold, Woodland Bellower. Unlike the rest of the high CMC creatures, bellower does not directly form a combo with any other creature in the deck, but was instead added as a creature tutor to specifically fulfil the role at 6 CMC. In this role, there were three main candidates: Woodland Bellower, Soul of the Harvest and Protean Hulk. Protean Hulk in fact costs 7 mana, however the requirement was only that the creature's CMC was 6 or higher, since the combo allows Yisan to untap at will, so you can simply fail to find at any given CMC and then go for the next one.
Ultimately Woodland Bellower was chosen for the following reasons:
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As Woodland Bellower is able to fetch Renegade Rallier, it still represents a combo creature. With either Blasting Station, Altar of Dementia or Greater Good this represents an immediate win condition. With Phyrexian Altar, once you have generated infinite mana you can sacrifice Saffi targetting Bellower and putting her in the command zone (which you can then recast with your infinite mana). You can then sacrifice Bellower to get either Elvish Visionary or Heart Warden and then go from there. The only altar which presents any issues is Ashnod's Altar. Here you need an additional colourless mana sink, via something like Skullclamp or Mentor of the Meek. While the deck does not run Augur of Autumn, this can be fetched with Woodland Bellower (assuming you have GW open to recast Saffi), which can potentially be used if there is no additional boardstate.
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Unlike Protean Hulk which has to both be hard cast and sacrificed, Bellower is one mana cheaper and only has to be cast, with no additional steps required. There is also no requirement for additional boardstate (in the form of a sacrifice outlet) meaning that ouside of combo, Woodland Bellower represents more value.
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While Soul of the Harvest can potentially draw you more cards, it requires that you both play it and then another creature. Bellower on the other hand both gets a tutor (making it easier to assemble a combo line) and doesn't require any external input to do its thing.
Ultimately the decision was made to include Woodland Bellower in this spot as it does more for less. however, any of the three would fill the role in the Yisan line and if you prefer to play one of the other cards intead then it is an easy swap.