oscillation
I read somewhere, maybe the POH of the 152 and 172, that the reason they mention precaution when slipping with full flaps is because of buffeting on the tail from the disturbed airflow from the flaps. Sometimes this could cause a minor oscillation in the vertical plane. However, I don't think it's limiting. And Also, I believe it's just full flaps that they mention, nothing about 10 or 20 degrees. I reguraly slip a 172 when needed with full flaps, but i have that in the back of my mind while i'm feeling for any oscillation.
Brian