Mandrel bending means that the piping keeps the same inner diameter/round shape throughout the whole exhaust, including the bends. The other option is press bending. Not any good, because when the exhaust piping takes a turn, it will be kinked and will create a minor restriction (but still a restriction). You don't want that with an exhaust, because you want the air to flow as easily as possible.
The only con I can think of would be, it may be a little tough to find a place that does mandrel bending. There's nothing bad about mandrel bending.
