As I vaguely remember from my days of flying Cessna singles (other than the L-19), the basic warning/prohibition was against full-flap slips, something to do with airflow off the flaps blanking the...horizontal tail? Been awhile, but I don't think it had anything to do with the slab-sided fuselage, since actually a standard 182, say, would have even more side area than an O-1/L-19.