If you mean the XDA developers AOSP roms, then they are most definitely NOT pure AOSP - Those guys are incapable of just building a rom from AOSP without adding their own cruft.
Not to mention that the XDA Developers site is impossible to navigate.
Not to mention that the XDA Developers site is impossible to navigate.
Contrast it with StackOverflow for example.
Also the tone is not very user friendly in general, I understand that when you post a rom you built you don't want to do support, but it can be pretty hostile.
The divergence from mainline linux, creating a new HAL interface, so much code and logic duplication which already existed elsewhere as FOSS software.