That’s because it’s the most egregious outlier in their kit. Not many classes have access to the ability to kill anyone in the game in under a second, but Convoke can do that. With pure variance.
It’s not so much a balance issue as much as it is an affront to basic competitive game design. It’s a spell that can do, literally, anything from nothing to instantly win the game.
Has more to do with how insane everything else is at the moment that Boomie is considered second rate.
Compare Arms/Ret/WW to Boomie, then Boomie to Hunter/DH. It’s actually insane just how good the former are, and how bad the latter are. Boomie just happens to fall in the middle.