I suspect maybe they weren't experts if Flask + React seemed to solve whatever the problems were. (Particularly since using React with Rails is fine.)

That said, I've encountered a solid number of Rails projects that have been dumpster fires because their devs didn't follow conventions AND had horrible modeling/schema issues.

(Example: let's create our own job system that's many times worse than Active Job for no real reason...)

The other recurring thing that I see come up a lot with Rails projects is that nobody can really agree on where to put their business logic for anything complex.