I believe you will get varying opinoins. Both engines are fine to develop for. Unity was probably chosen because there is already Unity content on some of the learning flows.
Unreal would do just fine as well. There are some courses here that cover Unreal as well. Do a search for Unreal and there is some content. I am a unity guy as well but mainly because I came across it first. And yes bolt is a paid add-on. I got it before Unity bought them. Super cool tool.
