On tandems the problem hasn't been stopping power for years (every since we had hydraulic brakes this has been a solved problem). The bigger issue on a tandem is that when you're going downhill you will end up overheating your brake, whatever kind of system you are using. There is just too much kinetic energy to get rid of as heat for your normal brake system to be able to deal with.
The kinetic energy depends on weight and speed. Cargo bikes easily come into that range, especially the fast 45km/h electric ones. Energy dissipation depends on the size of the rotor and sizes have been increasing substantially. When I bought mine, the standard disk size was 160mm, I opted for the upgrade to 180mm. The current generation of Riese-Müller Load uses dedicated cargo bike brakes with thicker rotors (2.3mm instead of 1.8) and a 223mm diameter on the rear wheel, 160mm on the front. Those brakes have substantially more staying power before they fade.
Edit: Generally, developments for cargo bikes seem to help tandems and tandem developments benefit cargo bikes. There's now an option to use the tandem version of the Rohloff internal geared hub for RM cargo bikes. That's a serious upgrade to all previous internal hubs, especially since you can combine it with a belt drive. It's a tiny bit expensive, though...
They why would you comment on what it's like to go downhill on a tandem?
I've got a Koga TwintySix, a trekking tandem, fully loaded it gets up to 180-200 Kg (two adults+camping gear, the 200 Kg is the maximum allowed load), the brakes are super good and yet, by the time you reach 70 kph downhill they are no longer capable of stopping the bike, safely or even at all.
A drag brake is a must in a situation like that or your brakes will start fading within minutes during speed reduction (short bursts of braking to reduce your speed to something safe and allow for ample cooling), and long before you really need them.
