conspiratemus1

The Achilles heel of same-side drive is fouling between the two chains in the event that the front derailer overshifts the main chain off the big ring (for a setup with the sync chain outboard) or the small ring (for a setup with the sync chain inboard.) In either case, you are left not with just a dropped chain (which if you are slick is often recoverable without stopping.) Instead you have two chains jammed together, one of which is under substantial tension, locked together by the humps and valleys in the contours of the two chains. You will be immobilized (in the heat, humidity, and mosquitoes) with one of you sourly standing by while the other of you, also sourly, attempts to unjam the two chains using a large thick screwdriver (not commonly brought along as an on-the-road bike-repair tool) after loosening the eccentric. You must also diagnose and correct, flawlessly and immediately, at roadside, whatever mal-adjustment caused the derailer to over-shift, else the problem will recur the very next time you try to shift the front as you struggle home.

The version of single-side we tried several years ago used the approach described by John Allen on the late Sheldon Brown's website (which Mr. Allen is maintaining.) The difficulty is you can have hundreds of flawless shifts, especially on a double, week in, week out, and you think you are pretty smart...then one day the derailer adjustment screw retracts that last fraction of a tenth of a millimetre and the overshift wrecks your day. (In my case, the chain jamming occurred on the first (and only) test ride. Fortunately stoker was with me, to provide moral force against ever trying that again.)

My suspicion is that, like much of the Internet, single-sides are only "vapour-ware": concepts that look do-able and "should" work, but are never actually built and tested in real life, even if claimed otherwise. The nice photo of a 42 bolted outboard of a standard double or triple crankset looks so clever. Not. The only way I can see a same-side being reliable would be if the aft sync ring was bigger than the the outer ring, and mounted far enough outboard to make room for the outer cage plate of the derailer, yet not so far as to interfere with the crank arm or with stoker's ankle bone (medial malleolus) as she pedals. A similar argument might apply to putting a large aft sync ring in the granny position of a triple crank, but then the chainstay gets in the way and being in Switzerland you might not want to give up a traditional granny inner ring. (I admit I didn't read every word of your post and I don't know if Di2 is immune to over-shifting.)

I do recognize the advantages of same-side drive. Our tandem with stock cross-over drive chewed up a new cup-and-cone bottom bracket during a two-week mountain tour in Austria in 2006 -- the balls came out looking like lentils and American/Canadian footballs -- but continues to behave to this day with cartridge BBs. The advantage of being able to use ordinary cranks, with their greater variety of lengths is a plus, yes, although my stoker can't tell the difference between 170 and 165 on our two tandems. But unless you have a sure-fire way to prevent over-shifts (and the certainty of chain jams), they will prove your undoing.