Every class has a fit within a group run and when people don't play to the strengths of the class it can be frustrating. But it isn't just bears or mages. Birds have a roll to play too.

Honestly, people use the word "tank" way too loosely in-game. I think the good bears lead the party and focus on keeping mob aggro and crowd control by grouping mobs in concetrated areas. Bears that don't concentrate on this area are frustrating for me to play with at times.

Mages have double duty, heal/rev and aoe mashers. Healing even when only one in the party will need it or to counter-act the negative effects of a spell (such as the slow spell in Nuri's that is canceled by a heal). And using combos. Bad mages can be just as frustrating as bears that don't do crowd control.

And birds have their issues too. I ran with many a bird while leveling my mage to 61 that simply would not use thorn wall appropriately never giving the chance to nature strike and who wouldn't take advice on the subject. So all classes can be frustrating. Not just one over the other.