Bearings vs. bushings in rear derailleur jockey wheels - which should be more resistant to contamination? This question came up in a forum I frequent. As background, many of us are familiar with ball bearings, as they are used in many places on bikes: the bottom bracket, the hubs, the headset, etc. Not all of us may be familiar with bushings, which look like this image from the Wikipedia link:



Basically you just put an axle through there. The axle rotates against the bushing. Bushings may not be as frequently used on bikes, but they are often part of MTB suspension components, and they may be used in your rear derailleur. Bronze has been used for bushings in other applications, but on bikes, plastic bushings appear to be more common.

This test by Ceramic Speed showed that some rear derailleur pulleys (aka jockey wheels) rotate on bushings, and some rotate on ball bearings. In particular, they showed that for Shimano's 105 rear derailleurs, both pulley wheels use bushings. In Ultegra, one wheel uses a bushing and the other uses a bearing. In Dura Ace, both wheels use bearings. And Ceramic Speed measured about half a watt of friction savings going from 105 to Ultegra, and the same from Ultegra to Dura Ace. So, perhaps bearings have lower friction when not contaminated.

However, some of the posters in the thread said that in their experience, bearings wore out faster, or they could seize in a muddy race.

The question is: are bushings actually more resistant to contamination than sealed bearings, or do they operate better under contamination than sealed bearings? If true, can anyone explain why this is so? In cycling, this is potentially a big deal if you just regularly commute in bad weather, plus it obviously might be a concern in mountain biking, cyclocross, or gravel.

I'm aware that you can clean both bearing- or bushing-based jockey wheels. However, I have to be frank - I basically just replaced the pulleys when they were worn. I bet many riders do the same. That said, I do intend to pay more attention to cleaning those things out now that I have a gravel bike, and to do so on my road bike also. This post on SE asked about cleaning jockey wheels, and the poster referred to the innards as bearings, but the picture does seem to show a bushing, as indicated by Nathan Knutson.
I've replaced bushings with bearings in derailleurs I've had and noticed  improvement. I've had bushings get gunked up and bind or freeze, and have had bearings do the same, but over all when my derailleur is clean, there is less friction and the benefit is noticeable.

Derailleur pulleys, like all other parts of the drive train should be kept clean. I do most of my riding on trails, so gunk is an issue. I occasionally pop the cage off and wipe everything down, spritz with some citrus degreaser, and reassemble, about every fourth or fifth time I pull my chain off to clean it. 

I think bushings are cheap and useless. It makes absolutely no sense why any company that manufactures bicycle components would sell me a drive train component with bushings rather than a sealed cartridge bearing.

This is a fun question. I've thought about this quite a bit. This one area that I base my understanding completely on my own experience.