So your first dozen sales are really just training
The biggest danger is that almost every deck sideboards Karn, rather than that Karn just beats everyone, and I think theres a good chance that wouldnt force them to ban Karn, although theres a small chance that that would push the price of Karn high enough to chase some people out of the format.
This has been the deck Ive been most successful with in Alchemy so far, and I highly recommend it for climbing the ladder
It does, however, affect each opponents' life total on any individual trigger
Its a neat way to bypass combat tricks, where your opponent could remove your enchantment to avoid taking damage
As it is, its really good in squirrel decks like Chatterfang or Hazel, and you can even conjure it with Euru, Acorn Scrounger on MTG Arena