My previous job was working for a .NET shop that powered tons of automotive search engines etc. across the web. We moved to more and more functional concepts using C# as the team learned about them and had a lot of success with them. At a certain point, we explored using F#, but the lack of type classes & HKTs meant we kept having to repeat ourselves.

We ended up choosing Scala for some new projects despite having an inferior IDE experience and being unable to use some of our other libraries. I don't think I can emphasize how much not having type classes and HKTs hurts productivity once you buy into the fully functional mindset. We were extremely productive in Scala because the language was a bit more powerful.

I would love to use F# again, but not until the language adds those features.