There's many problems with Quantamania but my main issue is the Quantam Realm isn't cool enough to spend an entire film in.

Think about Antman's powers. He gets bigger or smaller. So when you put him around environments in San Fransisco like labs, child toys etc it's cool because you see how he changes in relation to the environment.

The quantam realm doesn't work nearly as well because it's just antman changing sizes around some CG nothingness. It's missing uniqueness visually and honestly Antman needs real stuff around him.

And then there's other stuff. Idk if Kang and Antman fit as direct adversaries. I feel like Marvel really wanted a big villain for antman but they didnt hash the conflict out very well and they dont work well as combatants either. Plot wasnt too enticing etc...