I'm sure they're WAY safer than nitrous out of a cool whip canister, but I also used to own one of these:
You can put three full NO2 cartridges into one balloon (though I think I've only seen one or two people ever finish an entire 3 cartridge balloon before either letting go and wasting it, or face-planting). I KO'd myself on several occasions between the ages of 18-20. I've been told that during a few of these bouts of unconsciousness I've had tonic-clonic-esque spasms and had my fingers start turning blue. I'm not sure solvents are that much safer considering these possible massive dosages.