You cannot join some servers because the host is either behind a NAT or firewall and hasn't opened the port Gusanos uses, or they are using a different port all together.
The current server menu is very incomplete, however the next version will filter servers you cannot join and change ports to join servers with different ports.
If you get disconnected right after connecting, chances are that you have a half downloaded map in your defaults/maps folder (“Couldn't load map or mod” is written to the console). This happens when you join a server that runs custom maps, and the connection broke down while you were downloading the map. Go to the defaults/maps folder and delete the mapfolder in question, then try to join again.