It should be clear right away why Umezawa's Jitte is banned in Modern: it does too much
Simic StOmPy Might Have Been Too Repetitive With Llanowar Elves, Elvish Mystic, and Gilded Goose all at the one, Simic StOmPy could play a hyper-reliable first couple of turns
Its ultimate is an emblem version of Warstorm Surge, though you could cast that card for six mana anyway and forget the hassle of getting enough counters on the planeswalker
Our secondary combo is Isochron Scepter imprinting Dramatic Reversal plus mana rocks that tap for at least 2 mana for infinite mana and infinite instant casts, which again is infinite damage/draw with Niv-Mizzet, Parun
The decks in Pauper are not well equipped to handle an onslaught of huge monsters in the developing stages of the game
While it must occupy your active slot to get its healing online, I cant think of many Grass decks in the TCG that wouldnt be better without a lumbering shield that not heals itself every turn, but also packs a punch