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.
I once had to fix a Rails project where the original developer chose not to use ActiveRecord, ActiveJob or any of the Rails built in features. Not sure why he wanted to go that route unless he wanted to learn Ruby at the customers expense.
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.