Is the Remington 11-48 12 gauge semi-auto shotgun reliable like the 870 express magnum?

In my experience it's not quite as reliable, since the recoil action (requiring the entire barrel to reciprocate along with the bolt) is much more sensitive to differences in load. It also takes more time to cycle than a modern design would. That said, if it's in good shape and the load has enough kick, it should work just fine.