Hydraulic systems are usually a cinch to bleed etc...until the seals fail. If hydraulic brakes on bicycles are an example to go by, the small scale nature of the systems doesn't make for a design conducive to easy and effetive seal replacement. Hydraulic cycle brakes are generally pretty robust, but when they do fail they're a ballache out of proportion to the relative simplicity of repairing cable actuated systems. I see no indication that hydraulic shifting systems would be any different in that regard, if not worse.
Manual gearboxes on cars have remained almost entirely rod or cable actuated. If there were a more robust, effective, and cost-effective system the car manufacturers would have pounced on it long ago. So far they haven't.