Bill Tindall, E.Tn.
The problem Derek describes comes from too heavy a cut - just as you mention is the drawback
The drawback is I don't know how to deal with the reality of facing the need for a heavy cut. One can't just stick the chisel into the waste. If paring, it has to be removed in light cuts of some sort.
The situation is similar to what I deal with in making the dovetail sockets for drawer dividers. I am paring against a guide block. Indeed if I drove the chisel into the waste it would deflect. But in fact the waste is removed with arcing paring cuts using just 1/16" of the tip edge of the chisel. The waste removal goes remarkably fast-seconds. There is little orthogonal force on the chisel blade and the cut runs true parallel to the guide block. I suspect Rob does something similar but it was not obvious what. I'll ask if I'll ever get the chance for if this dovetail waste can be removed as quickly as the socket waste I have experience with Rob's method warrants further exploration. .