bondo inhabits that weird space between glazing putty and glass-reinforced filler that nobody really asked for, and isn't really useful in either situation.
IMO, if you can't beat the panel back into shape good enough to get away with glazing putty, you probably should just replace the panel outright.
Probably, but life doesn't always happen that way.
It works well if used correctly, which too many people don't seem to do.
Usually some combination of too thick, bad prep, and floppy structure/panel flex underneath it.