I have been using Godot since it was open-sourced in 2014. I'm also familiar with the source code and I have made some contribution. I mostly created prototypes until two years ago when I started working my first commercial project, it's called Outer Space: War Gears and it's a six-degrees-of freedom space shooter.

I chose Godot mostly because of the familiarity and hackability. However I spent a lot (too much) time working around/fixing obscure bugs in the engine. I figured I was definitely the first one to use some features (given they really didn't work), like using GDNative in a MT fashion.

Another issue (for Godot 3.x, Godot 4 is much better in this regard) is that GDScript is really slow. I got to a point where it took longer to run all the script than to run the physics step (and my game is physics-based, all actors are rigid bodies!). Luckily GDScript is fully parallelizable as it doesn't have a GIL. So now I run a lot of code in thread pools and do a lot of sub-step optimizations. If you do use it with MT, though, you won't be able to debug GDScript and profile on any other thread than the main one - for better profiling I just use perf.

However I'm reluctant to use Unity and UE because even just loading the project in the editor is much faster in Godot. I like it being so lightweight. I also like how you can easily write native modules for it. Recompiling the Engine is quite fast.

I hope Godot 4, with more support from companies and the community will get much better.