One of the best ways you can optimize your game is to reduce the draw calls, this is why Merge UV is an excellent feature of CC3 because you can reduce the number of materials in your character.
Unfortunately I haven't managed to make it work no matter what I do, I've tried with CC3 and CC3+, no clothes, no eyes and teeth, different skins, changed the normal map type.. 



Here's an example with the default CC3+ base male, I'm using UE 4.25 and have experienced this in 4.24 as well with a CC3 base character.
Creating the materials manually without using auto setup didn't help.



So far everything leads to a bad normal map but could it be something else ? Any suggestion or help is very appreciated, thanks.