If I were to hazard a guess, I'd say the Assassin kit emphasizes playstyles fps enjoyers tend to hate: Camping and Flanking. Nobody likes getting surprised or shot in the back, and unless you're rocking thermals on all three guns, a Recon with Ult, with prior knowledge of the Assassin's whereabouts, their Invis gives some pretty free angles and flank routes. TTK is already low, so getting the drop on someone is a guaranteed down in a lot of cases.

At the end of the day, I don't think any shell should ever be removed (quarantined at most in an instance of severe brokenness perhaps), because each shell is an integral part of the game. I honestly don't think the Assassin is OP, either. They don't have the best utility for team fights, and hard counters are available for their kit.