I've asked my gunshop/pawn shop owning friends the same thing,,,,what kind of trouble?
None of them have mentioned the gas shield.
They say the customer comes back, unhappy, with an 870 that won't shoot or won't cycle, or both. That's as far as they elaborate. They associate all the maladies with the heavy 3 1/2 inch loads. Which is, by the way, the only reason to buy the Super Mag,,,,so you have the option to shoot 3 1/2 inch super heavy loads.
These men are not avid shotgun addicts. All of them associate a sale of a three and a half inch 870 Super Mag with an unhappy customer coming back and wanting his money back, dealing with Remington repairs, etc. They all believe, right or wrong, that an 870 is one of the most trouble free designs ever made, unless you get the model that shoots 3 1/2 shells, and then actually shoot a bunch of 3 1/2 shells through it.
In other words, they are saying that the original 870 platform, designed in the late 1940's, could make the trip to handle 3 inch mags, but it's not a competent platform for the heaviest 3 1/2 mag loads. Too much recoil, too high pressures, cause things to break.
My own experience with 870's is they are utterly bulletproof. But I've never owned a Super Mag model, and if I did I wouldn't exercise the option of shooting 3 1/2 inch shells.