I use them regularly with MAPP. They work fine. Fixed lots of shifters, brake pedals, and other stuff. I never had a problem with dirty base metal, all I do is clean with scotchbrite and contact cleaner. I always bring some and a torch on multi day riding trips. Definitely worth it, practice a bit first though. Trick is lightly tinning the parts first.
The tip snapping off is no problem, You will replace it with a fixed type tip, not reattach it. You cut a piece of aluminum 1/2"x5/16" to the length desired or needed. Square the ends as well as the broken end of the pedal. Cut a piece of aluminum 3/4" x 1/8" thk angle to a length equal to twice that of the new extension piece. This will back up the new piece to the pedal. Tin all the parts with HTS-2000, clamp together, and braze/fill all the joints and edges. When cool, cut all but 3/4" off the top to form a tip. I use a small belt sander to finish things up. Works great, and can be rebuilt again as needed.