Bronzong may only be losing an immunity to one type, but with that one immunity gone it's losing a lot of versatility. It turns Zong from a safe switch-in to, say, Garchomp, into one of the worst pokemon you can switch into it. You may just be losing an immunity to ground, but most of Bronzong's useful resistances (Dragon, Rock, and Ice in particular) are frequently paired with ground, and taking that one immunity away really hurts it (actually, without that immunity, you'd be better off using Metagross as a physical tank).
As for what benefits it has...well, like other people have said, the surprise factor certainly helps. Bronzong is definitely weak enough that it can be beaten with just neutral attacks, though, and like other people have said it can't do much in just one turn.