Disc brakes can be noisy, they require more maintenance (pads don’t last as long), they can be a fiddle to clean properly and are heavier.
Yes and no. I've had rim brakes noisier than my discs. The discs do squeal a little but in my case it's because they've become glazed due to ridiculous amounts of constant light braking.
Maintenance? Almost nil. I use the road version of BB7's and in 2000 miles of near constant braking I've needed to turn the adjustment nob in a handful of times. Not exactly a lot. I've used brake cleaner on them once just as a precaution (glazing) and the only time I've had any cause to touch the cables was because of a brake lever swap due to going with straight bars then back to drops.
You'll be going through pad like cheese due to the nature of mountain biking. This is true of any brake system with the exception of drum brakes. The paste of water/grit/mud isn't kind to any material. In my case, I've had a set of pads last 2000 miles. Only now are they approaching changing time and there's still a good 1.5mm on them. Any less and there's a danger of them falling out from the calipers. And as you stated, rim wear is nil.
Cleaning. Again, nope. A blast of the hose is all that's needed as unlike rim brakes, no lasting deposits from pad wear are left sticking to anything.
Granted, there's a small weight increase over rim brakes and I do mean small. But in terms of pro's vs con's, weight is the only con.
The aesthetics of disc brakes can be subjective like anything else. I quite like them. Others don't. some (most it seems) cyclists like the look of black bars, seatposts, rims, stems etc. I hate it, prefering parts to be polished and/or silver. Each to their own.
I personally find that my mechanical discs are a bit more powerful than my previous v brakes and is one of the reasons I swapped. That and rim wear.
Most of my rides are doggy walking rides and as my doggy wouldn't take kindly to being dragged by the neck down a hill at 20mph, my discs are braked almost half the miles I ride. The last time I used rim brakes, my sputnik rims were worn to 1.5 mm within a year, new pads were needed almost monthly and at the bottom of certain hills my hands were near blue from pulling the levers so hard. A stroke I had 5 years ago took away a fair lump of hand strength which made that issue even worse. Disc brakes solved all that. A couple arthritic cyclists I'd spoken too agreed that their disc brakes also were a godsend.
