It is perfectly fine for some relatively simple things. Basic information apps, etc. But it isn't long in complex apps before the abstraction is leaking all over the place, and you find yourself fighting the tooling rather than leveraging the tooling. Which has been the case for virtually every similar "all platforms one tool" type solutions.
It worked between Mac and Windows on very complicated apps. It bridged Linux and Windows for tweaky MVC stacks a decade ago (then they wisely sharpened their focus.) It got the job done pumping data through hardcore game engines. The network stack is proven robust. You're talking about it like it's some stupid ORM wrapper or wonky Widget UI library. It is not.
Again, this conversation is about Xamarin the mobile app studio. There is zero ambiguity in this, so it is perplexing that you keep bringing this up.
The context is the cross-platform app creation toolset. It generates extremely poor quality code, usually at a significantly increased development time (quite contrary to the promise). This is the case found by almost everyone who uses it, which is exactly why most teams have an Android project, fully using the tools of the platform, and an iOS project, fully using the tools of the platform. If Xamarin were heavily used, Windows Phone wouldn't be so generally unsupported.
There is a language: C#. There are bindings to native toolkits. There's yet another imperfect Forms package. And there's a slightly wonky IDE. I'm not sure what you're expecting but I think the "lossy abstraction" here is mostly your expectations. I also think you are applying your narrow experience (which obviously was not a great one) and trying to amplify it by using unsubstantiated statements like "most teams" and "few wins."
BTW Xamarin Forms has or is about to receive a bunch of updates, perhaps it's worthwhile checking them out in case things have improved for you?
