I have a pair of friends that are serious tandem riders and they now run their premium touring tandem with Gates Carbon Drive. They switched from chain because, if you didn't know, tandem bikes with strong riders have a reputation for eating chains. (Premature stretch, leading to cog/ring wear, and eventually breakage because of the torque involved.) Belts are stronger than chains in some regards and I think high-torque applications is one of them. Witness Harley-Davidson switching to belts decades ago, away from chains. Also, on a bicycle, a belt system can shed almost a pound versus an equivalent chain-cog-ring-derailleurs setup. While being very quiet and very clean.

Downsides ... unlike an easy chain repair, you can't repair a belt in the very rare case it breaks. So you carry a spare on a tour. There are suggestions that frictional losses are higher with belt versus chain, though. And belts are apparently weaker when twisted or laterally flexed than chains, so storage and initial setup is more critical.