Guy, you may want to read this thread.
Pruning cuts - collar, collarless and co-dominant
After years of pruning cuts and watching the results I can confidently say that I have seen more stubs with poor results than flush cuts.
I can also say that on the trees I have cut here there's been near perfect results with even callus wood growth and little die back of stubs or aggressive callus wood on "stem tissue" from flush cuts.
The 2 most difficult cuts for arbos to do is the collarless union and codominant. The difficulty lies in knowing where to cut and recognizing the difference of the unions.