They are okay-ish, if you know the rules of the target code system/platform. (Ie. if you know React, and so you already create components in the UI builder.)

So basically my big problem is that these UI builders don't force/encourage the conventions that result in nice code.

The potential is there, of course, maybe even throwing a chatGPT-like AI in the loop can help with recognizing what the user wants and emitting and maintaining the resulting code base.