This. In my experience frontend is not for everybody. There is a reason why FE and BE are often separated - because it makes sense in most non-trivial apps. Related, I never understood the appeal of isomorphic apps either. It's not like the code for UI and data access is very similar.
It looks to me like FastUI is literally the BFF pattern with a pre-supplied front-end. If that's true, then complaints about it should be viewed through the lens of who has control over the application, who is gaining control, and who is losing it.
I've had quite vocal arguments from front-end devs which, when we stripped back what they were actually complaining about, came down to the front end being in service of the back end, rather than vice versa as they were used to.
