IMO their main problem is them being fucking slow. And sometimes AI (both ingame AI and "combat roles" AI) holds them at max range instead of getting right into the fray, even when you have aggressive AI selected and all your guns are 15 range). Sure, i can split my fleet into 10x20k and throw them at everything, instead of having one huge blob of destruction slowly running for enemy fleets (or usually i just let my vassals have fun with them while i conquer planets with my armies of mutants).
Well, also, if, for some reason, your battleships die it takes really long time to build new ones, so... if this ever happens to me i'll spam corvettes like crazy from all my active spaceports