This is the only post that mentioned Unigine and CryEngine - is there a reason for that. I.e. are there significant drawbacks to these compared to the "main" ones?
Unigine used to be very expensive, CryEngine difficult to master. In the meantime Unigine got cheaper and there was a lot of work to make CryEngine more accessible, so that might explain popularity of Unity (easy to learn) and Unreal ("golden standard" allowing AAA games with great developer backing).
I have mentioned UNIGINE cause I've heard that Dual Universe powered by that engine. It isn't free, which seems interesting, cause the best things aren't free. And best things and "widely advertised" and "common" are not necessarily the same