The torch is as hot as A/O is when burning. It's the control of how much of this energy gets into the work piece that's at "fault". Too hot a brazing will cook flux (preventing the filler to flow where the burned flux is), boil off the copper (resulting in a weaker filler), possibly change the base metal's structure into one that's less able to handle the stresses of riding. Running a hot joint makes the clean up take longer too as blackened flux is harder to remove then simple soaking off properly heated flux, not to mention the greater likelihood of filler build up where you don't want it (and the cosmetic need to file down said blobs).

Take these two photos as example. Both how after flux soak off but no other finishing The head tube brazings are fairly clean of blackening, show no copper run off. The seat stay top cap shows lots of flux burn but no copper boiling. This second brazing got a bit hotter then ideal. Andy.