Disc brakes are nice in the rain but have a whole slew of other problems... Overall I consider them more or less equal to rim brakes for midpaced road riding in all weather conditions.
You can't just say that-- you've piqued my curiosity. What are the slew of other problems we disk breakers need to be looking out for?
I mean, nothing that would be a deal-breaker, but just a couple things people don't usually think about. Here's the whole rundown on rim vs. disc...
System standardization:
Rim brake mechanisms are more transparent, easier to replace individual parts, and parts are available worldwide. Disc brakes usually use difficult to rebuild proprietary caliper parts so you have to toss the whole thing if anything breaks, if you're in a country that even has those parts.
Pads:
Disc brake pads last longer but are more expensive and are more susceptible to decrease in performance due to oil contamination
Adjustability:
Rim brakes are generally more adjustable, so squeals and chirps are easier to resolve
Rims:
Rim braves will wear out the rims after some time; how long depends on how much much abrasive muck you ride through. My oldest pair of road rims had 20k miles on them and were probably OK for another 5k when I sold them. On the other hand I know mountain dudes who like the mud and replace their rims every 10k miles or so.
Rim brakes also don't work well with out-of-true rims, although you shouldn't ride out-of-true rims anyway and could just use the rim brakes as a handy reminder to true up in that case. Disc brakes eliminate rim wear obvs, don't care if your rim is wobbly, and allow for lighter/stronger rim designs.
Rotors:
Can be bent, which kind of negates the rim truing argument in favor of discs above. Also more finicky to true back up, since disc brakes have tighter clearances and require a straighter rotor than rim brakes require of rims. Wear part that wears slower than pads but faster than rims, so they will need to be replaced every so often if you ride lots.
Wheel strength:
Disc brakes transfer braking torque through the hubs and spokes, which rim brakes do not. This adds a whole bunch of stress to the wheel, meaning beefier hubs and more spokes are required. You'd never know except that things will weigh more though, it's not like they're less safe. Just need to put a bit more metal there to make up for it.
Discs also require space for the rotors, so the spokes are set narrower on the hub, requiring more dish in both wheels. This does make the wheel weaker/less stiff laterally compared to a nondished wheel of equal spoke count, and there's nothing you can do about it.
Frame strength:
Disc frames/forks need to be a little beefier on the non drive side to handle the heavy forces applied through the calipers. Seen by users as heavier components.
Dry stopping power:
Both will make you skid. Once you hit this point it's impossible to apply more braking torque, so although disc might have more available braking torque, it's useless on the bike.
Wet stopping power:
Discs win here. Although an iron oxide rim pad like Kool-Stop Salmon makes a HUGE difference for rim brakes. I never wish I had more stopping power in the wet. Doesn't mean other people don't want more though I guess.
Heat:
Can lead to blowouts in rim brakes at continuous application at ~800W. Discs can handle ~900W continuous before rotor warping, brake fluid boiling or hydraulic seals melting. Disc brakes also transfer heat to bearings through the hubs so that's not good. These cases (for both rim and disc) are both SUPER RARE and pretty much mean you're using your brakes wrong, so don't worry about them too much.
Commuterbro accessories:
Disc brakes usually are in the way of rack/fender/kickstand mounts
Lever feel:
Mechanical disc brakes feel awful because there's nothing soft between your fingers and the rotor. With rim brakes you get a little compliance in the pads, and in hydro discs, that compliance comes from the brake fluid. The feeling of modulation between nice hydros and nice rim brakes is about even. Mechanical discs again are worse.
Other:
The position of the caliper relative to the dropout leads to a tendency for hard braking to pull the front wheel out of the front dropout with disc brakes. Not an issue if you're on tight and that's part of what the lawyer tabs are for, but it's still a thing.
Bikes running hydraulic discs can have a problem with air getting into the system in some weird cases, such as if the bike is flipped upside down or if you squeeze the levers without the rotor between the pads. This is very bad and means you won't be able to stop very well until you bleed the system, which is expensive and a PITA.
I think that's about it, I'll update if I remember anything else.
