> This is false, Unity is very popular, I've played several professional games made with it, for example.

This is true. Just look at statistics about which game engines were the most popular on Steam: https://steamdb.info/tech/

It's the most popular game engine out there by far.

Here's the top 10:

  Unity      29797
  Unreal     7665
  GameMaker  3173
  RPGMaker   2141
  RenPy      1392
  XNA        595
  Godot      504
  Adobe AIR  405
  Cocos2d    358
  MonoGame   293