I really jumped on board the train back when Microsoft's sudden openness made them the #1 topic on HN for awhile, and C# suddenly found new life as a cool fad language.
I tried out .NET Core on all three platforms, started reading C# books, and even listened to the weekly "Community Standup" video podcast to be updated on the implementation's progress. I believe certifications are a bit of joke, but I was even thinking about pursuing a MCSD so that I might shift my career more from Java to C# with my next job change.
I had realistic expectations about .NET Core's implementation in the early going, as it was Beta and rapidly changing. But I grew disillusioned as it went to RC and and even RTM, and was obviously still at the alpha stage in reality. Completely changing the build toolset post-launch? Seriously? And everything is branded with a different name every time I turn around?
The documentation and community was never on the same page with the current status of the implementation. I could just never get even Microsoft's own basic examples to reliably build and run. Perhaps the "Hello world" console example, but not anything beyond that. I would put it down, and every 3 to 6 month pick it back up again to see if I could get the ASP.NET Core example to run. Sometimes I could, sometimes I couldn't. And it was always one or two more API's (e.g. Entity Framework) away from being ready for real professional use.
For awhile I was checking in every 3 months, and then every 6 months, and at this point I think I'm on an annual schedule. And currently I'm no longer seriously considering a career switch.
I don't mean to be too harsh on the PEOPLE behind .NET Core, as they seem to be smart, good people striving to re-invent a platform and extend its lifespan by decades. But I think that some realism is sinking in now that the initial hype wave has subsided. Re-implementing even a usable subset of a platform as large as .NET was always going to take WAY longer than people were originally supposing.