Well for one you don't have access to the full range of controls like you do with split audio/midi
When they were first introduced they were exTREMELY buggy (for me) and I had a hell of a job getting one of my projects back to normal after they totally screwed up the track numbering scheme.
There were discrepancies between Track & Console view, gaps would appear in Track view where the "missing" tracks should have been. It was a mess.
But I haven't seen this behaviour in X2. I don't use them regularly, but I'll use them sometimes, when testing or debugging for other people.
Like now, for instance.