I'm only semi-back. I've actually made the jump to Unity (a while back) Ported all my projects over there, etc. Seeing as all features are free now, was kind of hard to pass up.
The entity/component model they have is slightly irritating, but I've found some work-arounds to it using a bridge monospace class to reverse the GameObject creates and owns component classes. Their model has some cool uses for rapid dev & integration into their IDE, but it makes it VERY difficult to structure code. So, I figured out how to break their model lol and reverse the process. Class creates GameObject attaches bridge component and stores itself within the bridge, which allows you to bypass their visual scene dev and work directly from a coding standpoint, but still leverage the benefits of their model.
Anyways...enough about Unity.
Has anyone here heard of Nakama server from Heroic Labs? They have a java client, which made me think of jMonkey. It's an open-source back-end solution for anything from mobile games to mmos.
If no one has mentioned it yet... go look it up and then start sharing it here on the forum. Literally the coolest thing I've found in years.