Other than the tanks, as mentioned before. I think it might also be AST's lack of mitigation.

I'm not talking about, like, Noct shields and Bole cards either.

WHM has an AoE stun (Holy) and SCH has an AoE slow (Shadow Flare). These actually make a noticeable difference on the amount of damage happening to your tank. At 52 AST's only AoE is Gravity, which has no added effect like the other two.

It's definitely a flaw with AST. It can make healing a bad tank really hard and it makes doing any level 50 content really annoying (I mean, unless you REALLY wanna cross class and spam Bliz II. XD).