I don't personally like playing AST, but I am of the opinion that the best party comp right now is diAST/SCH simply because of how much raid utility the cards and the fairies give. However, it's also true that Astrologian has a higher skill cap than WHM because it's slightly less reliable and requires much much more micromanaging with cards and royal road and whatnot. Not to say AST isn't reliable, but WHM is just easier with the higher potency and bigger range.

It's true that NoctAST can't really compare to SCH as a barrier role, but I don't think that's entirely a bad thing per se, since AST has the cards regardless of their sect. In any case, the thing that makes SCH stand out the most is the fairy, in my opinion, and the piss easy MP management the class allows.