I finished the Order campaign (only with Imperial Guard) last Sunday, and I sweated a little to complete the last two missions. Then I started with the Disorder campaign, and the first two missions weren't very difficult, but the third one started to stress me a bit XD
So I went back to the Eldars, to play their last two missions, and heck, the fourth one seemed impossible! It will take me a while before I come back to both campaigns. I hoped there was an easier difficulty level, but there's not. Probably there's a trick in each mission, as in IG's last two missions. I'll take a look.
Anyway, I appreciate Relic's attitude with this expansion pack. Two small campaigns, 4 different races to play, and something very original (at least to me) as playing as two races in the same mission, even simultaneously at some point if you want. Very cool.