The Sram Eagle (or the older 1x11 drivetrain) is very useful on a mtn tandem or mtn singles because it's virtually impossible to drop the chain on rough terrain, especially with a chain guide. It's also easier to find the right gear on rough terrain. A mtn biker is much busier than a road rider (pick the right line, shift weight, brake at the right time, absorb bumps, etc) that simplicity is a real plus. For instance, the left hand can control the dropper seat post while the right hand does all the shifting.

On a gravel or road tandem, the Eagle would be too expensive, and the gear jumps would be too big. For example, the Sram XG-1275 and XG-1299 (10-50T) cassettes retail for $220 and $450, respectively. By comparison, the Shimano Ultegra CS-R8000 cassette (11-32T) retails for $80.

I love my Sram 1x11 drivetrain on my mtn single, but have no interest in using it our road tandem or singles.

BTW, the Eagle is slightly noisier and slower than a 2x11 drivetrain (due to larger gear jumps).