I think cordless impacts do well if you are using plain impact socket on short bolt. Lug nuts etc. Or if you are showing off on youtube with your torque tester.
But if you add some flex into the equation like say 3" long impact hex bit socket or few inches of extension they fare much worse than their air counterparts. Possibly due to different hammer mechanism. (less momentum per blow).
For example, my cheap Ryobi (rated 150lb.ft) can remove lug nuts torqued to 100 lb/ft even bit more. But use it on long bolts with 3" long 12mm bit sockets and you can easily out-power it with one hand on short 3/8 drive ratchet. Happened yesterday.
I use it also to take apart ZF transmissions (ZF16S series) one such thing has dozens of 10mm bolts to remove before it spills out the internals. Without exception the Ryobi removes most of them, but it always fails to remove the longest of them. The reason is the long bolts flex enough to eat the tiny blows from it's hammer. It only takes 60 NM to remove them most of the time. I have measured it!
Needless to say any 3/8 air impact removes them with ease.
I don't have any experience with the latest and greatest 1/2" drive cordless units, but this summer my Aircat 1250K destroyed a brand new 3/4" drive 18v Milwaukee Fuel in unexpected torque battle. Possibly for the reason mentioned above, or perhaps it was a dud, or perhaps they are just over hyped. I don't know.
My point is; cordless vs air is a bit complicated when it comes to max torque in real world situations. And honestly I am extremely sceptical any cordless impact on the marked can replace the air heavy hitters in great many situations.
