You can create great apps with any language, and you can create buggy apps with any language. In other words: the choice of language has nothing to do with how buggy an app is going to be.
Flutter only makes sense if you need to create both iOS and Android apps, AND you can justify having developers learn a language (Dart) that is otherwise little used. For Android-only, Kotlin is the language of choice.
I haven't upgraded to Flutter 2.0 yet but I don't think it's mandatory to use hex instead of Material's constant colors. You've probably used a conversion guide that prefers to use 0xff. Portability is maintained, there are a lot of projects in Flutter that use Material colors instead of hexadecimals so it would be a big mistake not to be able to implement them.
Surely you will find info on the Material website. Trying costs nothing, we write the code but a thorough reading of the various because it is always useful.