I've only heard that in context of A: missing tooling, B: Microsoft, in their quest to make customers feel C# is still the real language and F#'s just some science toy. I've used F# for a Web UI via WebSharper and it was lovely. It's just a poor reflection on the state of UI kits if they don't work with F#.

"It's just a poor reflection on the state of UI kits if they don't work with F#." - well at least they don't start using lower-case names randomly.