The Super Magnum is no less reliable or smooth than any other Remington shotgun.
The Express guns are Remington's "budget" guns made to sell at as low a price as possible to compete with cheaper brands like Mossberg.
In order to lower the price, the Express guns are not as smoothly finished inside or out.
As they're used, the Express guns get smoother.
I haven't heard anything about the Super Magnum having any special reliability problems, but a lot of people are reporting extraction problems with all guns when using cheap ammo.
If the Super Magnum had some special issue with reliability, Remington would have fixed the design years ago.
In fact, the Super Magnum is simply the standard 870 with an action lengthened enough to handle the longer shells. The design is the same as the 3" guns, which is the same as the old 2 3/4" guns.
Where the Super Magnum "fits" is for use with steel shot on waterfowl.
Since steel isn't was effective as lead shot, the more shot put up, the better. The 3 1/2" shell simply contains more shot.
One work around, is to use the heavy shot high density non-lead ammo.
This is made from various heavy-metal alloys that don't have toxic issues like lead and are heavy enough to reach up for high flyer's.