There's things you can do in game engines to try to match Marmoset, but Marmoset does have some advantages and does not need to be optimized to run an actual game.
Its definitely possible to make it look even better in unreal for example then in Marmoset. You have access to a lot of things that you don't in Marmoset, however; it takes a lot more know how to actually do this.
Replies
These were both done in unreal for example:
https://www.artstation.com/artwork/ner4
https://www.artstation.com/artwork/gwqwE