I guess that R&M choose the Sunrace solution because there was no other solution to go under 11t at the time they designed the Birdy Touring. Other manufacturers like Moulton designed their own solution with proprietary hub and cassette.

The problem is that you cannot mount a chainring bigger than 55t or maybe 56t on the current Birdy models with the current chain tensionning arm. So the only solution to have longer gears is either a smaller than11t cog or an internal gear hub.

FYI, there are XD-XDR road cassettes up to 9-42 from E*THIRTEEN (XCX E*THIRTEEN cassettes seem to be road XD cassette, E*THIRTEEN is also the manufacturer of the 3T 9-32 cassette XD-XDR).

SRAM discontinued the Dual Drive several years ago, that's why several bike manufacturers using Dual Drive switched to the SA CS-RF3.

The choice between internal gear hub and derailleur is a question of taste, but its a fact that it isn't possible to switch gear while applying force on the pedals with the Alfine hubs what creates a lost of efficiency especially when shifting while riding uphill. The Rohloff allows to change gears while applying force on the pedal but the rotating grip becomes harder to turn.

The SA CS-RF3 add about 1kg (the naked hub without command and cables is 1090g) to the bike on top of the derailleur system, so not an option for people looking to a lightweight folder.