1. The plugin spams chat with 'Please, Set Your Password Using /register <password> <password>. It's quite obnoxious to new users. Possibly implement the feature where this chat pops up when you try to move or perform an activity.
2. The plugin seems to ignore the lobby location and so will end up wiping your survival inventory and xp bar on any world you're on.
3. Passwords are stored in plain text. Passwords should never be stored in plain text as this is a security risk. Passwords should be hashed and salted before being stored. I'd suggest watching the YouTube video by TomScott called 'YouTube Doesn't Know Your Password'.