Expo this, expo that, exp
Electric impact has come a long way. I've got a Ridgid unit that I really enjoy, makes wrenching super fast and nearly effortless.
The small 2cfm compressor I've got won't run an impact continuously. But something in that size range is enough. With the built in 3.5 gallon tank, it's good for a 4 - 5 second burst before performance starts to drop (plenty to pull 5 lug nuts or do most other jobs). And it recharges pretty quickly. I've also run it with an additional 7 gallon tank hooked up to it while working on the Jeep outside and with that setup, I've never needed to run it for long enough to drop the tanks below 90 psi.
In my case, anywhere I'm going that I'm going to want the impact, I'm going to have a compressor with me for tire pressure adjustments anyway. And I like having the 120v compressor, as it's got enough kick to have a chance at re-seating a tire after a de-bead. A typical small OBA setup with a very small or nonexistent tank would be useless for that.
I have a couple of the highest output Milwaukee 18v impacts , i make my living as an industrial service engineer with them. Any fastener on a vehicle that you would fix on the trail....no issue. Any large diameter fastner a pneumatic impact will always win . I did run a 1/2 drive electric impact off of the iverter built in to my 16 4 runner...super short bursts it worked...i mean short
You don't need 5+ CFM to run an impact unless you're planning on hammering away for minutes on end without ever stopping. 2 - 3 SCFM from the compressor is typically plenty as long as you've got a few gallons of tank. That won't keep up with constant use, but you'll usually be stopping for at least a few seconds before you've pulled the tank below 90 psi anyway unless the tank is tiny.