I have had an Atmos (7.1.4 with dual subs) setup for 3.5 years now. I have a ceiling that is below the desired Dolby spec at just under 8 feet tall. My only real option for good sound dispersion and coverage were to ceiling mount "on-wall" M3s using Axiom FMBs so that I could get the correct angle forward and back and be able to point the speakers towards the listening area. Works amazingly well and I've be in a good number of Atmos demo rooms at CEDIA since 2014 when Atmos was announced for home use.
Now, in-ceiling M3s have an aimable tweeter that helps to get better overhead imaging, but for really low ceilings, you still need the sound to be able to hit all of the seats, so to speak.
So in actuality, the further away the speaker, the more dispersion that it creates by the time it hits the listening area. The closer, then the less dispersion.
If in-ceiling speakers have a lower dispersion than regular or "surface mounted" like my on-walls, then you would actually want to only use them in high ceiling applications (ok, higher than my 7ft 10in ceiling, lol) as they would have enough sound dispersion at a lower ceiling distance.
Keep in mind that Atmos speakers are often referred to as "overhead speakers" when they should never truly be "overhead." They should be 35 - 50 degrees in front or behind the main listening area. That is why you wouldn't want something that is a tad more "laser focused" at those angles on a lower ceiling.
With all of that said, I have no idea as to the sound dispersion capabilities of Axiom in-ceiling speakers. If they are wide enough, the math may work out in one "lower ceiling" room vs another. For me, I had 2 reasons for going on-wall but mounted to my ceiling... 1) I have a lot of soundproofing technology in my room construction (and acoustical treatments, but those are movable). I would never be able to find locations for in-ceiling speakers that wouldn't hit a ceiling joist or hat channel or both that would fall within Dolby spec, even if I did, retro-fitting in-ceiling speakers would massively compromise my soundproofing. Additionally, I wanted the best Atmos sound I could get with Axiom speakers, so I needed a no-compromise approach with the speaker type and installation, that meant on-walls angled towards the MLP.
So that is what I did and it is outstanding.
Here is some additional information from an actual professional about angling speakers for lower ceilings due to Atmos dispersion requirements...
DOLBY ATMOS: DISPERSION REQUIREMENTS FOR CEILING SPEAKERS
