To me, the issue is how they design the games. Most open source shooters tend to make the players move extremely fast, with lacking weapon variety or balance (EG. all the bad weapons are the rifles, etc., and the explosive weapons are the most overpowered). There just really isn't much satisfaction in killing other players in this kind of setup (it would be a lot more fun if the gameplay were more refined) and they just feel sloppy in general.
I think TeeWorlds is the only open source shooter I've actually enjoyed playing, to some extent.