Reason why ClassAct/Reaction never really took off is they never understood what made MUI so popular.
MUI is nice, but it's also ridden with annoyances. just to name a few:
* Too bloody many options, why do I have to configure each and type of "box" or "list" or whatever individually for each and every class? Why is there no cascading of styles? "Apply this to all string input gadgets"
* No overview of which MUI apps that have individual settings, have to check this manually in env:mui/envarc:mui, and also (iirc), no way to set MUI settings for a certain app back to default settings (other than deleting the corresponding file in envarc:mui/env:mui
* No quality control of third party classes whatsoever, no official "approved" list, no warnings from the system when well known broken classes are called upon.
* Legal situation that only favours MorphOS.