The frame is extremely stiff. I believe this to be a requirement for safe high-speed driving.
Brakes are a problem on longer downhills.
You have to brake as little as possible, and to stop every few km to allow the rims to cool. With these brakes I do not go into larger mountain ranges like the alpes. Fitting a good disk brake could solve the problem, and I will try this in the future. On of the most common problems on tandems are broken spokes. Tandems exert huge loads that bicycle wheels are not designed for.