There have been multiple projects where lack of a library or database driver have resulted in hacks or passing something to a little Java app and back because it's unrealistic to do the work in C#. Like I said, the MS world seems to be an echo chamber and the vast majority of C# devs shun any solution outside official MS channels. Why do they do this?
Because open source devs generally still dislike MS and until .NET core (which is not ready for prime time) using C# saddled you to Visual Studio, Windows, SQL Server, and IIS. Because of this, the open source community on other languages like Python or Java is maybe 20x bigger.
The other reason open source avoids the MS platform is because they like to take your stuff and replace it with the "MS official" version. Entity framework was originally a poor emulation of NHibernate. WebApi a copy of Spring Boot. Instead of helping your project, or making it official like Java tends to do, MS likes to replace your OSS with something proprietary.
The other big .NET problem is speed. I like C# syntax better, but when I wanna get something done on my own I reach for Java first. It's faster and more flexible. In some areas the execution speed difference is 100x. The CLR might be quick but MS never considered performance a priority until .NET core, because before core they were only competing with themselves.
The echo chamberyness is leaving the MS stack in the dust. Everyone else runs on Linux now. Everyone else is using containers and SPA frameworks and Postgres. In the MS world, the unofficial rule is that if it's not a visual studio project template don't use it. It encourages reaching out of the Microsoft ecosystem as little as possible.
Big sites like stackoverflow that have chosen to go with C# have blogged about their adventures. Most of these adventures involve building things that are already available from the community in most languages. Core may save .NET but it might be too little too late. Windows has already lost the war for server operating system and that leaves little reason to use C# over the primary competitors Java and Go. Both of those have ecosystems far larger and far better support for things like databases, mapreduce, Linux, containers, etc...