Some *A airlines - TAP being the most notorious one for my travel patterns - won't pay for *G to enter third party lounges when flying in Y.

So, if you are accustomed to Oneworld like me (where there's no doubt at all - you can even choose your preferred OWS/E lounge) it's really annoying having to do research before booking a *A flight. The problem may not be all that widespread, but it's still there...