I built one tandem for Phil discs F&R only, no other brakes. That would not have been my recommendation, but the customer was Mr. Phil Wood himself so I didn't think I should push it... For the record, it never came back to have cantilever posts brazed on. (I think it actually was a show bike, not sure if it ever got ridden).
Of the probably two (maybe three) dozen bikes we sent out with those brakes, I only remember hearing of one or two failures; none after the newer discs with the reinforced spline area were fitted. I still wouldn't rely on one today, due to the lack of spares and not knowing how well those fiber discs deal with aging.
We did have one team that buckled their front wheel (taco / potato chip / pretzel depending on your geographic region). When the disc is crammed into a 100mm front fork it results in a dished wheel, weaker on the disc side due to the narrower center-to-flange spacing. This poor tandem team rode their brand-new custom bike about 10 feet and buckled the wheel before they got off the sidewalk, an inauspicious first ride. IIRC we switched them to a rear disc and rebuilt the front wheel symmetrically.
I think Phil did offer a symmetric F disc hub, but the fork has to be built for it. Was it 105 mm? 110? Someone here will remember.
Seattle, WA USA