They're big and bulky. The semi-auto response isn't great unless you go for the lipo/mosfet version, or DIY it. Power is way down at about 200fps / 0.37J, under half the energy of a GBB. They're nowhere near as fun to use as a GBB.
Full auto is nice, but you'll want the big 60+ round mags to make the most of it. And at the short ranges where you'll be using a pistol instead of a carbine, you'll be discouraged from using it anyway.
Consistent (low) fps, and no gas are the main advantages. But in terms of smiles-per-minute, GBB wins by miles.