The problem with in-wall speakers is that you need to be 98% sure that's where you want them for the next 10+ years. I really like the flexibility on-wall offers and the much smaller footprint of moving a simple speaker wall plate if I ever choose to relocate them (drywall patching a .5" hole is much easier than a 1'x3' hole!).
I've seen major living room installations with in-ceiling flr and rlr speaker configurations with a true center channel and though adequate for everyday TV watching, I think, if you care about music and movies, you will be disappointed with an all in-ceiling installation.
Don't get me wrong, I have some fantastic in-ceilings in my kitchen and dining room and they play really loud (powered by a Sonos Connect:Amp) but it's obvious the music is coming from above and not truly stereo even if I stand/sit directly between them as the angles are too harsh.
If you can swing it, I think you would be happier with an on-wall lcr and go in-ceiling with your rears and surround and height(?) for ATMOS
