I don't remember the specific boss fight, but it was a scenario where the boss went invulnerable and you had to kill a bunch of stationary orbs in a circle across the entire stage as fast as possible. It's pretty much the only time my summoner has felt useless. I recall other fights with multiple stationary targets as well, but I've been doing most content as a healer for the fast queues.

I'll admit it's a fairly situational problem; and maybe it's one that doesn't come up in a lot of new content. Either way that's the mechanic that feels like it breaks the class.