The big drawback with C# is being handcuffed to a Microsoft ecosystem, and will still probably be for a while. I also wonder how the Java ecosystem of tooling and available libraries looks compared to C#.
As others have said the open source community for Java is still leaps ahead of C#. It's not often you find a library which doesn't have a Java connector/implementation. For C# on the other hand you tend to be much more limited in your options.
I guess this used to be a lot due to the platform dependence of C# (and probably still is). Hopefully .Net Core can help with that, but it's not ready for prime time yet IMO.