The TRP Sphyre mechanical is best suited for single riders who are less than 200 lbs or ride at moderate speeds. Hydraulic disk brakes is the industry standard with mechanical brakes relegated to entry-level bikes. Since tandems are considerably heavier, it's hard to understand why tandem manufacturers would even spec a mechanical disk.
The Hy/Rd is much more powerful than Sphyre and it's only a $130 upgrade (F & R). Since the Hy/Rd is cable actuated (with hydraulic calipers), it compatible with non-electronic shifters. The Hy/Rd is also easier to maintain because the hydraulic pistons auto-center.
The Ultegra hydraulic is even better than the Hy/Rd (10-20%) but is more expensive and harder to install (eg. extra long hydraulic hose). It's even more expensive if you want to combine hydraulic disk brakes with Di2 or eTap shifters. A minor issue is that the hydraulic brake line makes the Di2 shifters slightly more bulky. The newer ST-R8070 may have a slimmer fit than the ST-R785.
As to reliability, I've never heard of a hydraulic disk caliper failing. It's possible to over-heat a rotor if the rider has poor braking technique (eg. drag the brakes). Remember that disk brake is an established technology, and has a successful track record over many decades on mtn bikes, motorcycles, and cars.