I dislike in-ceiling speakers due to the 'voice from above' experience they provide. It's fine for whole house audio/distributed audio installations, but is too severe a compromise on listener envelopment in an HT.
Few people here have a dedicated room. My back walls are all over the place, so my surrounds are mounted a bit too far off angle. While I plan on custom speakers, you may look out for different designs, di-poles for example, that may offer you a better sense of envelopment.