never heard of ue4 itself having core issues
some games yes, but not the engine itself
besides, up until like a week ago, gmod couldn't use multiple cores
in terms of mapping it would be 89125.3% easier in unreal
plus, code for the gamemode itself could be added to the engine, not tacked on with a secondary language
okay, the multi core issue must have been pulled out of my ass or some shit, but i swear i heard something was fucking up with the way it handled processors
also about gmod not supporting multiple cores, it doesn't need to, source engine is one of the easiest engines to run, ever. almost any computer can run it with decent graphics
the mapping would be so much easier. hammer is, by modern standards, heavily obfuscated, difficult, and of course out of date. plus ue4's mapping tools are really good. no doubt about that.
the documentation for the code in ue4 is really bad. like really, really bad.
have a look at the docs for ue4 c++ here:
https://docs.unrealengine.com/latest/INT/API/
and compare it to unity's here:
https://docs.unity3d.com/ScriptReference/
you can see the amount of documentation for ue4 pales in comparison to unity's, unity's is much more clear cut and easy to use, along with a UI that isn't cancerous and hard to navigate
ue4 has a lot more power, but it's a lot more raw than unity
of course, ue4 has blueprints, which anyone with a brain and some time can use (it's a bit more to it than that, but it would open up addons and modifications to a larger group)
i love ue4, but it isn't perfect (and it would be badass to see a gmod type game implemented in it)