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.
Contrast to languages where there isn't a spec, or the spec is the interpreter, such as Perl 5 (Perl 6 actually has a specification and multiple implementations or various levels of conformance), or things in between, such as Python with the Python Language Reference. There are benefits and drawbacks to a standard/specification. One benefit is that it's easy for someone to start their own implementation and test for conformance and have a high degree of surety whether it will work with existing programs. One downside is that when there's a problem found in the spec, it often requires more work to fix, as the changes need to propagate out to the implementations, which will have their own timelines as to when they can implement it.