"Why is Zombie and Mr. all the way at S above everyone else? They're just for combos and aren't usable as units"
Zombie and Mr. get significantly more mileage than any other purchasable special because of their combos, and it isn't even close. The sheer power of research combos absolutely outweighs the utility of the lower placements as units since Research combos directly contribute to the viability of units like Ramen, Courier, and Rock who is arguably the strongest unit in the game because of both Mr. and Zombie cat.
"Why isn't Riceball at #1 regardless of uber combos? He is straight up a better Manic Eraser"
This is only true if you're also accounting for a maxed cashback or cost down dojo orb which is not included in evaluation due to their rarity. At 300 cost, Riceball is less efficient than Manic Eraser. Riceball offers 97.2 HP per 1c opposed to Manic Eraser's 170.3 HP per 1c, making Manic Eraser 75.21% more efficient in defense. The tradeoff is that Riceball has immunities, but in nearly every general scenario it's actually Manic Eraser who outclasses Riceball which is also why I recommend using him over Riceball when crafting your own general/ perfect lineups.