I owned a share in a C172 with 40 flap for about 10 years and never really got to the bottom of this.
Only issue I ever noticed was that you got a fair bit of buffet over the tailplane when sideslipping with 40 flap. I only ever practiced this at altitude as with 40 flap it dropped like a brick anyway, if you needed to sideslip also you'd have to have made such a pigs ear of the appraoch you should be throwing it away for another try.
Only reason I ever tried it out was to keep it up my sleeve in the event of a forced landing as a 'last resort to get you in'.
Heard all sorts of conflicting rumours from the tailplane being blanked giving you no pitch authority (not the case when I've done it) to the buffet potentially being confused with stall buffet etc
Interested to hear if there is a definitive answer....?