Would in-walls or floor standing speakers for your main channels be the better option? Yes, but doing all in ceiling still works well.

The cool think about Atmos is that it tricks the ear into hearing the sound from wherever it needs to be. Kind of like how my old Bose sound bar tricks me into hearing sound from behind me.

It's all smoke and mirrors anyway, so install the speakers where your customer wants them and let the Atmos processor do it's thing. It's not always going to be optimal.