We've had a Rohloff Thorn tandem (straight bars) for nearly ten years now. Did our honeymoon (Strasbourg-Saltzberg) on it and this year will have a go at some Audax. We too have joked about the S+S couplings being divorce-friendly.
Regarding the gear range, we find it fine. I can't remember what cog/ring base ratio we went for, but it was a little higher/longer than Thorn suggested (they are kind of the Volvo of bikes, IMO). If we get to a 20% hill with full touring kit then we're walking, but that's rare. We do sometimes spin out in top gear, but we're touring, not racing, so it doesn't make sense to go all out down hills.
If the range is really not enough for you then you can swap the chain and cogs for a one off. I've no idea if this is practical with a belt drive, though. The other (pricey) option is a
Schlumpf drive which doubles the number of gears (no doubt with some overlap).
We're not at all bothered about the gap between the gears, but I've been used to riding a hybrid 11-32 8-speed for 14 years. In any case, on a tandem any gear is a compromise between the two riders, so neither of you can expect to be at your "perfect" cadence for much of the time.
AFAIK derailleurs on tandems are the same as on standard road bikes, so I'd expect them to wear out much faster. Unless you have some kind of special harmony that we haven't yet acheived, I'd expect more the the gear changes to be "messy" ones with more load on the chain than ideal. With a Rohloff this can stop you changing, but it won't wear things out. (In practice I tend to momentarily force-slow the pedals as I change). The only true annoyance is a slightly sticky 9th gear - changing up into this sometimes gives what feels like 12th, before it drops back to 9. I can cope with this as I'm aware and it's still a great set up.
Maintenance is lower, I'd say. The oil change takes me less time than a full derailleur chain-cog degrease and clean and I only do it once a year. No reindexing or tweaking, seems to "just work".
One of the best things about the Rohloff on a tandem is surely the changing when stopped. If you stop in a high gear on a solo bike, you just lift the rear wheel and spin one revolution to change down. My wife's light, but I can't lift her and the rear wheel one-handed! Coordinating us both getting off and lifting it is quite a faff - oh, quick the lights have changed!
What I'm not convinced about is how well it would work on drop bars. I tried out Thorn's Mercury to see and felt that the shifter, being on the tops, was too far from my hands when in the drops, hoods or bottom bit. So I ended up riding in the "wrong" gear a little too often as it was more effort to shift than on brifters. I only had an hour test-ride, but I imagine after all day reaching up to shift it would be quite wearing!
Overall it has some quirks, but on the tandem I wouldn't swap to derailleur whatever you paid me.