Paul, I use park assist all the time. The issue for me was that I was in a parking garage with a concrete island next to my car. Knew it was there when I parked the car, but upon leaving 8 hours later I had forgotten (don't usually park in that garage).
Park assist will not aid you in determining what is parallel to the car, only in front or back or at oblique angles front or back. My front bumper was ahead of the curb to my right (as an island, it wasn't like a long curb attached to a road), so the sensors registered nothing when pulling forward (I curbed driving forward, not reversing, so mirrors and rear sensors would have been of no use). That, and the fact that the infotainment system display took probably 30s+ to boot up after the car was turned on (dark screen, no info, nada -- this is the second or third time that's happened, adding a bit to my distraction).