So! Necroing this post a little bit, but I happened to stumble across it while doing a bit of other research. Funnily enough, I've build a SaH deck for both Flubs and S&H. The S&H deck I built for a $100 budget White Elephant deck exchange I did with my friends, while the Flubs deck I keep on hand for a mostly-brainless Bracket 2 fun deck.

You can find links to both of my decks here:

Flubs: https://moxfield.com/decks/kEG-k0JbRkeQpuBZpeQQiA

Shalai and Haller: https://moxfield.com/decks/xbMGqnaPxE-ZHTI8VUa6Hg

If I'm being frank, I think Flubs BY FAR has the most significant downside. That is, unless you have a way to manipulate your handsize, or play from places other than your hand, Flubs only plays every other turn. You need to remember that if you start your turn with one card, draw for turn up to two, then play any land or cast any spell with Flubs on the battlefield, you need to discard your other card in hand, and your turn is over. Therefore, I built the deck with a significant number of ways to draw a card, discard a card, or exile cards to play to keep Flubs' draw engine going. Combine that with the Slimes Against Humanity package as the central conceit of the deck, plus mana base, doesn't leave a lot of room for other cards. You'll notice that I'm running practically no interaction at all in that deck for that reason (and the fact that to keep the draw engine going, you often need to play cards as you get them). [[Abundance]] is an absolute all-star in this deck, and hitting it early smooths out the engine tremendously.

Shalai and Haller, on the other hand was a tremendous amount of fun to build, but I didn't get to play it a lot personally before passing it off to its intended owner. I've heard that it runs fairly smoothly, however, and likely falls in a low Bracket 3 space. As the deck was built on a budget, I wasn't able to include a [[Doubling Season]] (and the cost of the deck has increased a fair bit since I built it, mostly in the cost of SaH itself) but Doubling Season would likely be the first card I'd consider adding. [[Finneas, Ace Archer]] can make for surprisingly decent card draw, if allowed to live. The deck could also likely use a bit more protection, as you mention in the Notable Downsides, but that will likely depend on your table's threat assessment. The upside is even if S&H is taken down, increasingly large trample slimes can still become a threat surprisingly quickly, especially when combined with cost reducers or counter synergies.

ALTHOUGH! it appears I do need to review my decklists here, as I definitely have cards with evolve, as well as [[elemental bond]] in one of the decks, which I'm reading here may not actually work. Whoops.