AdiIRC

I am not sure if this is a bug, but I suspect is uses a different way to get the nicklist data.
When I join a twitch server, my nicklist is empty except for me.

You can get some code with the viewer list using this:
https://tmi.twitch.tv/group/user/CHANNELnameGoesHere/chatters

replacing /CHANNELnameGoesHere/ with an actual channel name.
I don't know if that helps.

irc.twitch.tv
port 6667

For now, I just made an alias like "/people" and it echos the URL so I can click it to see who is in the channel.

I like using IRC for twitch, because it disables all of the emoticons (which are over-used, making them spammy).