With a conventional tandem with triple, I'm good from about 5 mph to 30 mph, and there's not much around here that falls outside of that range. (But, if it did, there still bigger rear sprockets, available, too, so you don't have to a da Vinci system to get lower gearing.)
I mentioned a friend had one. They love it, primarily for the independent coasting. And no, they don't go offroad. But that just doesn't strike me as a need. Yeah, if you had a bicycle where your right leg and left leg pedaled independently, somebody would like that, too, though.
On the "left derailleur"- very simple in concept. But tandems mainly use single-bike parts, they're already a small market, so there's a limit as to what you want to produce in the way of specialized parts for them.