itch.io

Actually, the API is very well documented, but due to the prevalent tutorial-culture, this is often ignored. Also people tend to claim things are "not suported" because they didn't find youtube-tutorials for them. (like realtime IK for example).
https://docs.blender.org/api/2.78b/#game-engine-mo...

Another big problem is that people, attracted by Blender tend to see logic-bricks and fall prey to the Dunning-Kruger effect (and some features are only accessible via the API). I'd claim, it's very much as capable as other engines (I'd consider the editors more powerful, tho), but - very much like using Blender as a video-editor - there are a lot of convenience features missing, that one has to implement first. Of course, if you're comfortable with reading manuals and working with APIs, all that could be considered a bonus to the already decently integrated pipeline. - You could in theory do everything but music without ever leaving blender. (and I'm sure people more inventive than me could use it as a sequencer as well ;) )

Edit: Heck, if you're comfortable with using python's OS library as a bash replacement, you could probably replace a whole desktop environment and most of it's software (save a webbrowser) with just blender. :D