Have been tandeming since 1975 and have used all kinds of shifting systems, including early version DI-2 .
Di-2 failed us in the middle of a hill climb without any warning after a recharging. Turned around, went downhill and pedaled home in one gear.
Most reliable we've found is barend shifters. Reliable, relatively inexpensive, and lasting over 50,000 miles without failings.
Hi-tech is fine and gets bragging rights and wow-factor.
Currently use a triple and 9-speed cassette (11/34). Reliability and longevity and as a bonus a decent price point are key.
Jus our input/experience.
Pedal on TWOgether!
Rudy and Kay/zonatandem