In the Discord client, nicknames for all users show up properly.
In the Ripcord client, nicknames only show up for some users.

Nicknames not showing up and colours -
Normal Discord Client example: https://i.imgur.com/yRrSFw3.png
Ripcord: https://i.imgur.com/jKOnuwe.png

Nicknames showing up for some with colours - 
Normal Discord Client: https://i.imgur.com/ZfSEQNR.png
Ripcord: https://i.imgur.com/Lk75bjj.png

Ripcord will not sync the full user list or user details on guilds with more than 4000 members. This is an intentional limitation I added after Discord transitioned to a new user syncing system which is incompatible with the multi-tab/window model of Ripcord. The old syncing system places more of a burden on the Discord servers (I presume), so I added an artificial limit in Ripcord to avoid causing problems for Discord.

I've already implemented the newer system earlier last year, but it only allows viewing the user list of a single channel at a time, so I have avoided making Ripcord use it in the UI. Eventually, Discord will likely completely remove the old system, and Ripcord will be forced to use the new one, at which point I will probably have to begin disabling the user list in anything except the active tab, and add a delay between being able to show/hide the user list in each tab.