I think C# is underrated because it was closed source for so long, and working with a closed source ecosystem is such a pain (and risky!). At least personally, that made C# a complete non-starter for me before. However, I've recently started learning it, and it really is nice. It's much more pleasant to work with than Java.

Hopefully it will see a renaissance when people realise that with ASP.Net Core they can write code that looks broadly similar to Rails/Laravel/Django code, and get much better performance more or less for free. There are definitely some rough edges around the ecosystem though. Entity Framework isn't nearly nice as Laravel's Eloquent ORM, and it's pretty jarring looking at a promising library, only to realise that it's a commercial offering (don't get that with PHP!).