Is the average .net developer terrified of F# because they're not talented enough, or because it's different?
One major issue I do see coming from the C# side is "well how do I do this then?!", which often the answer is "you don't, because you don't need to" or "well what if it's more performant to do it mutably!" well then thankfully F# can absolutely do that.
If you keep an open mind it's really a very clean and simple language, but in an age where half of development is importing 8 well known libraries, not being the main supported language is a major weakness.
However, I wrote my first thing a port of a small C# tool of a couple hundred lines, to F# in about an hour. It's been about a week now, and things are considerably smoothing out.
To be considered niche, the F# tooling has been great. Also, having the .NET libraries available adds a lot of built-in capability.