They are a lot lighter than most hammers, and perhaps less pleasant to use. Needing more wrist flick to compensate for the weight and feel of a fiberglass or better yet, wooden handle. But I have a problem right wrist and am right handed. But seriously, its the first set of dollies I ever saw with scooped out places on them. It kinda defeats the purpose of the tool, providing less inertia to back up the hammer. For working lighter gauge smog era GM steel, they probably do you just fine but as you just saw, if you swat the right spot and get good aim and contact, you'll pinch the metal thin in a spot and blop-pop goes the high spot.
As far as "mapping the contour" goes... Just act like you are going to block sand some bondo. Slap 180 grit on any sanding block and just slide it in an x pattern first one way then another, over and around the dent spot. The bright straight line scratches will stand out from the orbital scratches and make it easy to locate and evaluate the roof's contour discrepancies.