I had heard it was something related to the flux.Oh, and as for *why* brazing, it's probably because it puts less heat into the panel. Since it's two pieces of sheet metal butted together, it's probably more susceptible to warping than the overlapped gutter, and right at a spot where it would be obvious.
From what I've read, the problem people have had with primer and filler sticking to the braze is actually due to the flux residue. Body guys who advocate brazing recommend thoroughly washing the area with water. Probably not everyone's favorite complication when we have MIG welding as an alternative.
That's just my opinion from dabbling in bodywork, welding, and brazing.
I would think with a torch it would cause more warping, but IDK, I guess it's how you control the heat.
I used to be really damn good at brazing, but I haven't done any for 35 years.