I’ve been working with pure functional languages and custom lisp dialects professionally my whole tenure.
You get a whole bag of problems for a very subjective upside.
Teams fragment into those that know how to work with these fringe tools and those who don’t. The projects using them that I worked on all had trouble with getting/retaining people. They also all had performance issues and had bugs like all other software. You’re not missing out on anything.
Many problems stem from people not being willing to learn another paradigm of computer programming. Of course teams will split, if some people are not willing to learn, because then some will be unable to work on certain things, while other will be able to do so.
You mention performance. However, if we look at how many Python shops there are, this can hardly be a problem. I imagine ecosystems to be a much bigger issue than performance. Many implementations of functional languages have better performance than Python anyway.
There are many reasons why a company can have issues retaining people. A shitty uninteresting product, bad management, low wages, bad culture ... Lets eliminate those and see whether they still have issues retaining devs. I suspect, that an interesting tech stack could make people stay, because it is not so easy to find a job with such a tech stack.
However, many companies want easily replaceable cogs, which FP programmers are definitely not these days. So they would rather hire low skill easily replaceable than highly skilled but more expensive workforce. They know they will not be able to retain the highly skilled, because they know their other factors are not in line for that.
