I ran the Cooper Discoverer AT3 on my Grand Cherokee. They are great from highway to dirt roads, maybe dry forest roads, but not beyond that. They are not an offroad tire.
I don't care what the Editor of EP says, I speak from my experience running that tire.
Interesting you had that experience! We had the opposite when we had them on our JK, but I'm not surprised that there are differences like this. In our case, we found they performed better than the stock BFG MTs (We ran through two sets of those before the Coopers) in all terrain except for very watery mud, and in those conditions the M/Ts only had a slight advantage. We also found them much better in below zero temps. Lots of factors at play though -- gearing, power delivery, etc. that may make a big difference between a GC and a JK.
The main reason I linked the Expo article is specifically because it informed our choice, and because it wasn't an editorial -- it was an article that recorded a standardized testing process on common off-road tires so that there could be as close to a 'head to head' comparison as possible. For those seeking an objective answer comparing tires, a standardized, repeatable testing process is necessary; anecdotal experience like we both have with the Coopers makes things incredibly hard to delineate. There are a ton of factors at play, so many that even the individual vehicle is only part of it -- perhaps someone else with a JK will have the same experience as you did, whereas someone else with a GC might be closer to my experience -- but other factors mean that performance will depend on how heavy/light the rigs are, the terrain they are in, the composition of the dirt/mud/track, and even to some degree their preferred off-road driving style (some people are more aggressive/passive than others).
For any given application, I think there are generalities where if you pick a certain tire you can't go too far wrong, but that's only only half the battle. I think in order to find the 'best' tire, the tire also has to be matched to the rig, the driver, the weights, the terrain, etc. and the best way to do that is to toss the shoes on, and put on some miles!