some servers, such as irc.twitch.tv don't show users in the user list (at least they don't show up when using /names). And tab completion doesn't suggest them.
This makes it annoying to mention someone because tab completion cannot be used.
This could simply be fixed by allowing to mention users who have sent a message before.
however, this may also be a problem with the /names command because i have seen a user list in other IRC clients such as HexChat before on the same servers