I quite like Unity.
It's a good engine, BUT....

I despise doing UI in it, it's just so tedious to make everything work and show up.
In the past I didn't mind but then I learned about XAML in WPF, UWP and Xamarin. XAML works very nice with the MVVM pattern (even if you do that lazily) with the data binding.

So now I'm working on my own data binding, which is both fun and saddening (because it's not in there already)