Yeah UWP has a lot of good features but I always find when I go to it that it just lacks so many basics. It's like they kept building good stuff on top of Silverlight which stripped out a lot of useful stuff. Having said that I've not used it extensively (I had a good stab at Metro, but man that sucked) and ever since haven't gone back.
It's not x-plat either ;)
Obviously with Avalonia we have a long way to go (hence the Alpha) so we're getting the basics down first.
Layout on the UI thread is definitely a limitation too. I'd be interested to hear ideas as to how it could be done in a threaded manner.