And yet, as an example, every LDS (Latter Day Saints) facility has surface-mounted ceiling speakers and the sound in the area that's used for services has been equalized with very good results. When the actual speaker quality is considered, they're usually Atlas and are nothing special, the results are rather impressive. While these aren't music listening rooms, music is played in them and the coverage is excellent, speech is very clear and if someone walks through the space, there's almost no major distinguishable changes from one place to another.
Sometimes, it's not possible/practical to use box speakers, set up for perfect sound. He didn't ask for an opinion on in-ceiling speakers, he asked about room correction.