Nex 5N backfocus?

Yes, missed focus is a problem with the 5N in certain conditions (low-light on subject, plus lights in background). Although many people here seem to not have a problem, I do a lot of shooting situations like nightclubs that have the exact factors to fool CDAF.

CDAF is very reliable. It's just reliably wrong at delivering what I want in certain circumstances. At least these circumstances are predictable, so you can compensate for them or use MF with peaking.