The biggest problem (other than high cost and completely changing how I might design my circuits) is the space they eat up in a 20/40 or a 12/24.
I've long believed that it would be a better panel design that would integrate the neutral bus along side the non-neutral bus, and allow breakers to have 2 connections into the bus, instead of just 1. That would eliminate all the pigtails that panels are going to be full of in the coming years. But such a thing would be an all new standard most likely incompatible with what exists now. So it is unlikely any manufacturer would want to switch to it.
