I have used an M2 with XRail a few times, just not the fully integrated one. There was a learning curve, but we got past it pretty easy. Mainly changed out the operating spring for a lighter one, and had to run faster loads though it. Ran like a dream after that. Did have to be cleaned between matches to. Sharing the M2 with a friend at shotgun matches is what got me to want to add one to my gun. I had a Stoeger M2000, and sold it because there wasn't an XRail that would fit it. And that's why I bought the Mossberg over an M3000 or M2, since it was a gas gun and wouldn't have the weight issues inertia guns can have.
Mag fed guns, I have seen a few run great. But at the Super Shotgun matched we have locally every year, the mag fed guns seem to have tons of issues. Have only seen a couple of them make it through the match that did have problems. from what the guys running them say, it's usually a magazine issue.