The fact that you blow a TON of money on CO2 cartridges.
Plus, you've just got a trigger on the CO2 unit, not a full regulator that'll hold X psi...meaning you won't be able to carb to a set volume...it'll be a lot of guesswork and corrections.
Now, for taking a keg to a remote party, those CO2 chargers work great. You force carb off your 5lb (or larger) co2 cylinder, and then only use a charger to push beer.
I didn't keep good track but I got about 15 cups of beer off one 12gram bulb the one time I've used mine.
Primary: English Mild
On tap: Pale Ale, Lancelot's Wheat, English Brown Ale, Steam Beer, HoovNuts IPA
Bottled: MOAM, Braggot, Raspberry Melomel, Merlot, Apfelwein, Pyment, Sweet mead, Cabernet
Gal in 2009: 27, Gal in 2010: 34, Gal in 2011: 13, Gal in 2012: 10