Dunno what you mean. I’ve been using a Pixel since Pixel 1. The UI has only changed a bit at a time. I’ve used Messages (Google’s SMS app) for as long as I can remember. It’s only ever had one AI assistant, which is the “hey google” one, and it’s always been okay.
I’m guessing some of the issues you’re noticing are specific to some Android distributions from specific brands. Mainline Android is pretty stable.