I haven't really thought about it. Now that you bring it up, generally, survivors-like don't go for precision or skill (like aiming, shooting, timing, etc...) they're more relaxed games that can basically be beaten with WASD only. What I've seen in Costume Crash is that the attacks are mainly directional. For those kind of attacks I'm sure you know, survivors-like mostly aim automatically at the first enemy that comes in range and auto-attack. So... I don't know what to tell you, I didn't see any huge benefit of having them being manual really. You just have to ask yourself why. Is there a reason it would make the game better if they're manual? Are you expecting aim and fire skills and accuracy from the player or you want them to have a casual session after school/work? It's a design choice that unfortunately I can't help you with, it all depends on what you want from the game to be. But it's a big one, it can make or break the game.
If I had to decide, seeing that Costume Crash goes for for a friendly casual halloween aesthetic, I'd make them auto. But that's just my opinion really, it's all up to you. Hope it helps. Good luck with that!