It's true that .NET Standard 1.0-1.6 had a lot of gaps but these should be rectified with 2.0 now so you shouldn't run into these problems anymore (otherwise there's usually a good reason why a method/API is missing from the standard).
Disclaimer: I'm working on Mono for Microsoft/Xamarin, primarily class libraries and tools.
It's true that .NET Standard 1.0-1.6 had a lot of gaps but these should be rectified with 2.0 now so you shouldn't run into these problems anymore (otherwise there's usually a good reason why a method/API is missing from the standard).
Disclaimer: I'm working on Mono for Microsoft/Xamarin, primarily class libraries and tools.