.NET isn't, and wasn't officially meant to be. That's your problem.
I've been working on a .net core web app on my mac exclusively now for the last 8 weeks and have had no problems with it. I do check to make sure it works in full VS/Windows occasionally and every single time its worked without fail, and without a single change. Just pull the code and go. I haven't needed to touch windows but its nice to know it still works over there anyway.
Using VS Code as my main editor. Tried Rider a little yesterday and I'll probably switch to that once its more stable as it has a few more tools for refactoring etc.
So .NET core is clearly what's intended to be used cross platform, and it works great.