I did a few and I did not find them difficult, but then again, I live in a remote area and the schools were small. It went well. The people there were very nice and so was the person I had to phone ahead (for the appointment)
The photo audit doesn't take very long, but you need to wait around until lunch service starts so that you can observe how the staff greet/treat the students and take some service timings.
If it's the one I'm thinking of, they require me to order far too much food to be comfortably covered by their reimbursement. I hate when that happens.