Mitering is a piece of cake, it used to be regarded as a skilled profession, but the existence of tube mitering programs takes all the guesswork out, or the use of machine tools. The fact that bike tubing is so thinwalled means it is relatively easy and fast to shape with simple hand tools.
Brazing lugs is very similar to plumbing work as far as the principles being similar, clean, flux, heat, and draw the filler to the source of heat. But there are many differences. The torch needs to be used skillfully because the relative heat is extreme, the torch can easily melt the metal, and certainly destroy it. Also the lug is a lot more complicated in shape, and can have considerable heat sink built into it. One is making a joint that is required to bear structural loads, and using the bare minimum of materials to get to that end. There is also no easy test, like a pressure test with plumbing, to prove the efficacy of the result. I'm sure that virtually anyone can learn to braze, but it does take some skill to do to the level required in a fine frame.