The C172, as you say, has barn doors. These give a huge ROD. Approaching the ground below 1000' with a 2000 FPM + ROD in low hours hands is not a good idea. This is one reason.
The other is as you say....blanking.
A sideslip needs all 3 controls: Ailerons for the bank angle, Rudder to control the descent path and Elevator for speed control.
Some types are ok to slip with full flap, but not the 172, as the downwash from the lowered flap, reduces the effectiveness of the Rudder and Elevator.
You will also notice a speed change due to the pitot position (try opening the door....)
If you have too much angle of bank, you will need loads of rudder to keep on your flight path. This may cause the rudder to exceed it crit AoA. Don't forget the RAF/Flight Path is now in the direction of the lowered wing and 90 deg to the rudder keel.
Again the elevator has also a RAF/FP change. This blanks the raised section of the Elevator, as the aircraft slips towards the lower wing or surface.
Not good if the slipstream off of the wing surface is deflected down by flap, producing more downforce on the elevator and an increased load factor.
With it being blanked and at a higher load factor, you might not get a lot of warning when she departs controlled flight.