I'm not so sure that I agree. I have mechanical disc brakes and the brake cable stretches and I have to adjust it every 250-500 miles. If I don't, my braking performance will suffer, especially in emergency situations. Granted, with hydraulic brake lines, you'll have to bleed the brakes every so often, too, but I imagine that it's be less often (unless you live somewhere where your brake lines will freeze!).
Also, I find that even when my brake cable is properly adjusted and my pads are new and everything is adjusted properly, I get less stopping power with mechanical discs than I do with hydraulic discs. The modulation seems different. I can still modulate braking with hydraulic discs, but I seem to get more braking power than with mechanical discs.
I recently test-rode a Kalkhoff Integrale with hydraulic disc brakes and tried out some heavy braking just to see how it'd perform. I accidentally locked up the rear tire under heavy braking, and I didn't intend to do this. Whether that's a good thing or a bad thing is entirely subjective. Obviously, you wouldn't want to lock up a tire under heavy braking while cornering, but you wouldn't want to have too little braking force in an emergency situation, either. With my 2015 IZIP E3 Dash with mechanical disc brakes, I can't lock up/skid the rear tire even if I try. I know that in some states, there are laws that state that bicycles must not only have a brake, but that they must be able to skid the rear tire to show that their brakes work. Neither my road bike with caliper/rim brakes and 23C road tires nor my ebike with mechanical discs and 47C tires is able to skid on either tire. Perhaps my tires just have too much grip?
I'm solidly on the hydraulic disc train, and it's not due to the marketing. It's due to the braking power and braking feel, which I feel is not only superior to the feel of my mechanical discs, but is also confidence-inspiring.