Your question is based on the fallacy that game engines are that important. Fact is that game engines are tools. You don’t use a hammer to saw wood, you use a saw. Likewise, game engines are fit for different tasks. Some engines like Source are fit for first-person shooters, some are fit for small grid-based puzzles (Bitsy). Game Maker is fit for sprite-based 2D games. So is Construct 3, but more targeted towards non-programmers.
Under this analogy, you know what Unity and Unreal look like?
Hence why I’m not really fond of either. I find them way too broad and general-purpose.