Are disc brake post mount adapters compatible for both front and rear?
I actually can answer it now myself.
Shimano has "rear" and "front" adapter. This is a relict from the time where post mount adapters were not used yet on the frame and therefore post mount were only for the front.
See this chart from Shimano
Other brands like Avid/SRAM don't even distinct between front and back.

Post mount adapters are in general compatible for front and back, making the front/rear label obsolete.

Source:http://forums.mtbr.com/brake-time/rear-shimano-adapter-160-203-pp-can-i-use-front-mount-1083866.html