I'm a green card holder (UK citizen). Over the past month used APCs at MIA, IAD and DFW; BWI had them but they weren't being used.
IAD: Used same line as US citizens (both immigration and customs), no problem and quickly through.
MIA: same line as US citizens (both immigration and customs), got an 'X'. Was told it's because I had scanned my passport (as instructed by the machine), CBP said I should have scanned by green card (of course - I'm psychic!).
DFW: US citizens and permanent residents split at the APC - but no queues to speak of, got the 'X' again and had to do all the fingerprint/photo that I had already done at the kiosk with the CBP officer (i.e. double waste of time). At customs the lines were again split between US and Canadian Citizens, global entry, and ESTAs and permanent residents - so I was directed to the latter which took over 40 minutes to get through - which was particularly annoying since green card holders should expect the same treatment as US citizens.
As with most things in the US, the system seems to open to interpretation by whoever is in charge at the local level... .