I've just read a review in Singletrack magazine of Shimano's XTR Di2 system, which by contrast makes evident some potential (relative) advantages of a hydraulic system.
Firstly, the Di2 wires are thinner than mechanical cables (so, like hoses, you need different stops, I presume). I'm guessing they're similar to dynamo wires, which I've known to break on more than one occasion (in fact, I broke the wire from dynamo to front light of my wife's favourite bike just a couple of weeks ago
while refitting a basket I'd removed at some point in the past). Presumably gear wires run much close to the frame (even inside it) so are much less vulnerable, but it's still a potential failure. Hoses are going to be tougher, surely.
Secondly, the front mech is "a hulking great lump of plastic and metal" and the battery is "about the size of a small minipump" and occupies bottle cage bosses. I guess the hydraulic system would be slimmer and obviously it wouldn't need a battery.
Thirdly, they say that under load Di2 "still doesn't shift as cleanly as SRAM's system", but presumably this is a case of small degrees.
Fourthly, there is still a lot of noise. In fact, it seems it's noisier than a mechanical system because in addition to the click at the shifter and the noise of the chain moving across sprockets (which obviously you'll get with any derailleur system), there is a noise from the motor which they describe as "a distinct, robotic 'squelch' ... as the motor within each mech does its thing." They go on to say "it's really annoying".
OTOH, Di2 can be set up with any lever controlling either mech – this must be possible with hydraulics too – and in any shift pattern. For instance, you can set it up to give a sequential shift on one lever even if you're running two chainrings. I don't see how you could do that with hydraulics.
They sum the Di2 up as "the benefits don't come anywhere near outweighing the initial outlay and replacement costs". We don't yet have any prices for the Rotor system, but I think it's telling that we're all comparing it with electronic systems rather than mechanical ones.
