I set up many of my tandems with the Phil Wood Disc Brake without and problems. I would caution buyers that there was an "O" ring that held the disc in place on the hub axle. If the "O" ring was allowed to age to the point it could fall off, it would allow the brake to migrate away from the disc resulting in less than optimum disc to splined driver contact. The disc does require some skill to assure proper setup and adjustment. The disc should float freely so that there is no side stresses on the reaction arm where it contacts the frame.
Replacement discs are no longer available. In my private stash I still think that have one or two reserved for the original customers that I built tandems for.
The very earliest Phil Wood disc brakes had a friction disc that did not have the reinforced center section. These would be more prone to failure when not set up properly. Phil would modify earlier brakes to accommodate the newer design disc.
All the tandems that I built had a "pacman" tab on the frame and the Phil disc reaction arm modified so that it made use of the "pacman". http://www.classicrendezvous.com/USA/Phil/Phil_pics_1.htm