If I assume that the post your referring to is mine (I decided not to use the M3c for my Atmos ceiling because of concerns with imaging). My thinking:
Atmos is an object placement format. The movies sound engineer states “this object is at this point in 3D space and here’s it’s audio track”. Given that infomation each play back system (yours, mine, your local movie theater, etc) figures out how to get it there by using its unique speaker layout and locations. How close to the exact location will depend partially on how well those speakers image.
That said, at the moment Ian is probably right ... most of the time it’s just an overall effect, the exact placement does not matter. But the power of Atmos is when is does matter and I suspect as time goes on it will matter more and more as engineers embrace this format.
Ya, I don't know what the future of Atmos mixes holds.
Regardless, I'd be shocked if I could tell much of a difference between an in-ceiling and in-wall in the ceiling when it comes to Atmos. Especially with no a/b test.
At this point I am thinking, from an atmos perspective, given my 8ft ceiling height (which is on the low end), the amiable tweeter is going to be of more benefit to me than a speaker that has the driver slightly in front of the plane. Of course, I could be way off on that.
Either way I am confident things will sound just peachy with m3 in-ceilings.