Only real setback I see with it is like what was mentioned regarding boats. It is an almighty PITA to dive sidemount on a boat that isn't used to seeing divers in this configuration. That being said, if you have a chill boat that will support it, it works out great. I am still trying to get used to clipping the suckers off while standing or floating on the surface. I've kind of called it quits on my buttplate, and clip them to hip D-rings until I'm on descent, then I'll swing them back and get everything situated when I'm descending or at the bottom. This is mainly diving cold water with gloves though, so everything feels funny in gloves fighting gravity.
Other potential problems
Rigs are different to get used to, and may or may not require more weight-inability to use a hard backplate
The inflators are run funny. You either have a REALLY long inflator hose going up and over, or you switch them and use a wicked short one, not really all that big of a deal.
Regulator routing is funny, and for some people the switching back and forth bit is odd for them. I didn't have any issues switching over, and do it second nature not. I cross my regs behind my neck, which some people don't like.
Biggest one is really the getting in and out of the water on boats though, it's really a pain on some boats, so I throw doubles on for that. I don't have to truck them on my back very far and my knees aren't bad enough to where I can't climb up a ladder with 130's yet.
Bob has the best point for all around SM diving for me. I can grab a pair of any tanks and throw em on and have full redundancy which is huge, or just a long bottom time.
I dive Sidemount almost exclusively. I'll put tanks on my back with students in OWT, and if I'm kicking around a lot on the surface. That is all done with my Nomad though.