There are at least some Soma and Surly cogs that are made with narrow gullets (the gap between the teeth). With certain chains, the chain roller is slightly wider than the gullet, so the chain actually jams into the gullet and has to be yanked out by your pedaling action. At best it sounds noisy, at worst it locks the chain up in the cog and you just start skidding or you pull the wheel in the stay ends. I don't see the problem all the time, but this last year we've had too-frequent problems at the track with these cogs. It got to where we had a couple Dura Ace or EAI cogs around just for those who showed up with bum cogs.

Another issue is the quality and type of threading on the cogs. The incidence of stripped hubs goes up with cogs other than EAI, Dura Ace, Suntour, Sugino, and Phil. There are two different cog threadings available -- ISO and British -- and they are almost but not quite compatible. You can force one onto a hub threaded with the other, but you're eroding the threads on the hub and shortening the life of the hub. Sooner or later, with enough changes and/or enough force applied, you'll simply strip the hub. So you want to keep an eye on the choice of cogs and hubs. Even if they're made of a decent grade of steel, you have other considerations in the cog you pick.