I'm not even sure what "compatibility" means in this context. You can run executables for one on the other? But you can swap out the OS on the Pi, so... I'm just confused.
This kind of thing is what has really annoyed me about the Pi 'movement'. "Raspberry Pi compatible" means nothing. It's a totally standard ARM based linux computer, of which there are a huge (and growing) number.
It surprises me that Pi compatibility is a criteria for stuff, though I know it's popular. Are we talking pin-compatibility?
AFAICT the Pi is just another linux on a single board device. It's low cost but otherwise fairly unremarkable.