I tried it with no stinging at all, but it made my face red and splotchy. I liked the feel of it, but I don't think my skin liked it.
Seems there are at least two groups ... those whose skin benefits from the use of alum, and those who get irritation from alum.
Your opinion on the use of alum probably depends strongly on which group you belong to
It also depends if one uses ammonium alum or potassium alum. It also depends if applied with hot water or cold. It also depends if you leave it on or rinse it.
In my limited experience that is.
I get splotching if I do not rinse.