I've actually had to fire a technically exceptional Haskell programmer because of the damage they did to our C# codebase (and arguably moreso, the team). Sometimes it's not a matter of talent or skill, but culture fit.
In my experience FP-aligned people on non-FP projects tend to be more likely to overengineer, more prone to argue in favor of the Great Rewrite For No Reason Except Aesthetics, and more likely to abuse "lesser" programmers when they put up PRs. They suck as team players on teams that are not made of language nerds. I am not just talking about the one person here who I fired, this is a legit pattern I've noticed over at least a half dozen people.
Conversely, they are exactly the right people to deploy when you have really tough, self-contained problems to solve that you wouldn't trust the normal Java 9-5ers to tackle.
No matter how they do it, you can always rewrite their working code in a more maintainable language later once it's working, and make it integrate well with the rest of your stack. :D
