A cards legality in Pauper is based on if it was ever printed at a common rarity
The key differences are background requirements and edition accuracy
Magnus The Red Spawn Your Army Of Spawns Magnus the Red comes from the Warhammer 40,000 Universes Beyond set, and has been something of a sleeper hit despite combining two very powerful red and blue abilities into one card
Dismember, Shriekmaw, Umezawa's Jitte - Spot removal for creatures
However, if you do and all of the targets are illegal when the ability tries to resolve, it won't resolve and none of its effects will happen
I fully expect the price on nonfoil uncommon FCAs to drop dramatically over time, rares and mythics will still be hard to pull but there will be a fair amount more of the nonfoils out there, thanks to the Play Boosters