C# is hideous to get running consistently (if at all) with recent versions and libraries cross platform. Perhaps this will change in the future, but they've not been particularly brilliant about it over the past decade
I don't know what libraries you talking about but nearly everything from nuget starting with image processing and ending with orms and crypto work just fine on linux.
Even more when K will come out I don't think there will be any worries at all.
Just an example:
>Microsoft released C# Roslyn source code, announce Linux support with K Runtime this year
They're going to the right direction, maybe someday...
>Apple released Swift, closed source language that runs only on one platform (Mac)
Apple is so awesome, keep innovating guys! Now let's write a webstack and nosql db using Swift!