Not much, really. There are a few things to take into consideration, though.
They generally have a lower HP engine & less tractive effort, so they'd have to MU a bunch more engines which can be uneconomical (fuel, maintenance).
Lack of dynamic brakes.
Irc, air brake reservoirs on switchers are usually smaller. Not good for maintaining trainline brakes.
And if they're being taken on the road, the lack of crew amenities (no toilet, fridges, etc) could be an issue as well. Especially with collective bargaining agreements.