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.

Yes, that is a case of bad normal stitching algorithm. I got the same in Blender.
Got to have a cloth to cover it . Or take it somewhere else to merge materials. Or submit the problem to FT and wait for the fix

Thanks for replying, yeah that sucks, I already submitted it
https://www.reallusion.com/FeedBackTracker/Issue/MergeUV-to-UE4-normal-map-issue

Until then I'm just gonna try to learn how to merge the materials in blender.

That's nice, I only need to merge the body skins like you have in your image so I think I'll manage xD, many thanks.