The rocket science is in controlling the direction, and frequency spread of the diffusion. The width of the recesses and their frequency affect this. Also some diffusors have multiple different scatter patters going on at the same time to change the range at which they are effective.
No, they are not overrated, but may be too expensive, IMHO. ;)
What they do is give you a high density solution, which allows you to precisely tailor a solution. Meaning, for each square foot of wall or ceiling space you get a lot of effect. Also, diffusion, especially between the speakers, really improves imaging and clarity, not by deleting sounds but by increasing the volume you get at your ears in the right, diffuse manor that actually helps you hear more.