But is it widely used? Stable? Popular? Really nice to use? Actively developed? Is it infrastructure that other higher level tools build on, or is it like unity?
It’s a reimplementation of Microsoft‘s popular XNA framework that was used for a number of successful games. Monogame itself has been around for a while and my impression is that it’s a mature and stable project. I would put it in between libraries like SDL that are often used as infrastructure and full engines like Unity.
not by major game studios anymore but plenty of very popular indie titles use it, yes, yes, yes, yes, its a framework/sdk that comes with everything you need to make a game, though its more a code first model so it isn't "editor-centric" like unity.
