It wont be good just thrown in as is
When you mutate onto an already-mutated creature you just have to choose which creature goes on top
The fallout is bigger than one bad transaction: Collectors and players get ripped off
Considering that a card effect is the most believable way to take such a huge beatstick off the board, it will likely replace itself anyway
From an EDH perspective, Unstable Amulet is a payoff for casting exiled cards, and thats a small list, including Passionate Archaeologist and Flaming Tyrannosaurus, so Prosper, Tome-Bound and various Doctor Who decks will rejoice
Provided that the monster your opponent controls can be used as material for a Fusion monster in your Extra Deck, this card can easily remove an opponents difficult-to-overcome monster while fueling your own board and allowing you to bring to the field an even more powerful monster than what you had before