I can relate to the OP.
Right now I am dealing with this BS: Having issues with floors not getting same baked lighting as walls/ceilings - Rendering - Unreal Engine Forums
Since Epic advertises UE4 for mobile platforms and mobile VR, where lightmapping is a primary (and most of the times the only) lighting model, Lightmass should do the job 110% by now. Yet, it doesn’t. Ancient Quake 3 lightmapper does a better job than UE4’s Lightmass. And yet, Epic doesn’t care much. I think UE4 can be considered indie friendly only if indie dev working in the same alley as internal Epic’s projects (or in the same alley as AAA devs). Step left or right, and indie devs are screwed (unless said indie dev as indie as ID Software or Epic used to be 
)
I don’t mind discovering bugs and creating test cases for Epic to fix, if they fix it. After all, we don’t pay anything to use engine (and only pay 5% over certain amount if game/apps sells well). However, that’s not the case. It begins to feel like small indies are nuisance to engine devs, something that can easily be pushed aside. Especially when it comes to mobile / mobile VR for Oculus platform.
It just feels like such well established thing as lightmapping should have been resolved and stable by now. Something Quake 1 initially resolved in 1996 and q3map2 lightmapper for Quake 3 resolved in 2004 (it has indirect lighting, AO, deluxemaps, etc. almost everything UE4’s Lightmass has).
At this moment UE4 is great … for Windows and consoles only, only if you are using dynamic lighting and not mixing up indoors with outdoors, and only if your gameplay is in line with what engine already offers internally.