We do commercial work with unreal 4 but we only just reached a quality which is sort of ok to give to some clients. check this out.
watch?v=KkgrN-t6EkI
It is not super good and I see a lot of little problems. But it is used commercially. It is a side product of a real time walkthrough which runs on oculus rift on a decent framerate with a 980 video card. It runs on an average laptop on 20 fps without oculus.
I agree with what you are saying: it will never be as good as a corona or vray render but it is getting closer. If your client doesn’t need 8k output then you might be ok. There is room for both in the arch viz world. But primarily unreal is made for real time. Static images are better done in corona or vray.