I have no experience using them but received this comment from a reputable manufacturer when discussing the use of front wall quadratic diffusers in addition to absorption:
“ our take on QRD is no thanks. They scramble phase information which tends to
obscure sound stage detail and their energy storage can color the tone. The
practical effect of this is usually a more blurry sound stage, but with great
continuity across the room. But I must admit they look cool, and it's not all
bad, just a little more like an omni-speaker experience…”
Since I have Spatial Audio M4 Sapphires this comment was also offered:
“ as much absorption on the front wall as possible below about 300 Hz for
dipoles gives a glorious presentation. But you always want to be mindful of
keeping the treble range lively in the same area…”
It probably comes as no surprise that this mfg doesn’t offer such diffusers and we weren’t discussing rear wall treatments.
I also found it interesting that Real Traps offers far and near versions for rear wall applications depending on listening position proximity to the rear wall.
https://realtraps.com/diffusor.htm