It depends a lot on what you are repairing and how much of the original frame you want to keep. Some parts of steel frames can be easily debrazed. Seatstays, dropouts, shifter bosses, and cable guides are easy to remove. Repairing other portions of lugged frames can be easier than building a new frame IF you are willing to replace some tubes and/or lugs. It is difficult to debraze lugged joints without damaging something, but that is not an issue if you sacrifice one of the tubes/lugs from the joint. You have the original geometry of the frame to work with, so this can make putting everything together easier on you.

Carbon is extremely easy to repair. Slap some carbon and epoxy on and wrap it with electrical tape. The main difficulty for carbon repair is fixing the paint once the repair is done.