My aversion to cable operated disks is the same as why I don't like V-brakes on road machines- V-brakes and mechanical disks are designed to work properly with MTB style levers only. Road levers don't pull enough cable- you can test it yourself. MTB levers pull almost exactly twice as much cable as a road lever.
To make V's and mechanical disks work with road levers (read that as "STI" shifters"), you have to resort to devices like Travel Agents and so forth which work by trading mechanical advantage for length of cable moved. So you take nice strong brakes that work fine in their intended setting and force them to work with road levers by effectively making them weaker. Totally bogus in my opinion, and a triumph of marketing over reason and the creation of a need where none previously existed. Hah! I'm enjoying this rant so much that I forgot what my original point was.....
And, to add insult to injury, you've got that long run of stretchy cable going to the brake brake, meaning it hardly works to begin with, then you put something on it that makes it work even worse. That makes sense.
I'm really an advocate of hydraulic brakes for road tandems, and I agree with Christophe Timm that the Magura HS-66 system is probably the best brake system that you can put on a road tandem. Unfortunately, they preclude the use of STI, which makes them out of the question for most people.