Depends on the gun, from what I know. Once while I was in training, we went through about a week of **** in the wilderness - rain + dust + shooting practice = very, very dirty bolt and carrier group. I actually wasn't aware how full of dirt the thing was until I took it apart... never had it jam on me, not once. Needless to say, I was very impressed. That was with one of the new G36s. On the other hand, you'd be lucky to find an L you could actually finish an exercise with, without getting a jam.
I haven't fired a DEagle, but from what I've read, it seems like it's fairly prone to jamming, especially if the shooter isn't experienced and doesn't deal with the kickback well. 9mm guns don't compare in terms of kickback to the magnums, so I wouldn't know, but it doesn't sound too far-fetched. And the conditions in combant aren't exactly the same as what you get in the shooting range, either.