Air Asia is reliable in the sense that they rarely cancel flights, but evening departures being delayed is the rule, not the exception. Krabi queues can also be pretty bad, they don't field too many international flights so it takes a while when you get 200 people and 2 "mai pen rai" Thai immigration officers. No visa needed for Thailand, just queue up and get stamped in.

All in all, instead of stressing out over this, I'd strongly recommend staying overnight somewhere in Krabi, with a nice dinner on the beach in Ao Nang or something, and taking an early morning boat instead.