I'm inclined to suggest it's pretty futile trying to guess what is actually happening to the flow; what comes off those barn doors will be all over the place, and the way it interacts highly variable.
That said, I'm inclined to think if we had stalling tail feathers and 90 degree pitch excursions, there would have been a few incidents, and something would have been done. There are enough 172's around, and they date back to when the majority probably regarded sideslips as normal, we're hardly talking about a hot new design.
For comparison, with C-dog, my 172N, manual says, (on page 4-19) "Steep slips should be avoided with flap settings greater than 20deg due to a slight tendency for the elevator to oscillate under certain combinations of airspeed, sideslip angle and centre of gravity loadings" Emphasis mine.
So yep, it varies by model. Hopefully we've also answered why!