WhatsApp is a hugely popular messaging app with over 2 billion users. It's easy to use and the vast majority of rebels will already have it installed on their phones. Does this make it a good fit for your XR group?
Security 
Despite being end-to-end encrypted, WhatsApp has a number of serious security issues that make it a poor choice for serious organisation in XR. To start with, it is owned by Facebook, a company whose income depends on collecting people's personal information. You can read WhatsApp's privacy policy to get an idea of the sorts of information that they are collecting.
Another serious and often overlooked security issue with WhatsApp is that its end-to-end encryption often does not work. Most WhatsApp users enable an option called 'Chat Backup' so they can recover their messages in case they lose their phone. If this option is enabled, for even a single person in a WhatsApp group, then that means that all of the group's messages will be stored, unencrypted, on either a Google or Apple-owned server, freely accessible to the authorities.
Can be compromised if malware is installed on the phone or computer, or other people have access to that device if they use your login. Or if you have infiltrators - no app protects against those.
Accessibility 
WhatsApp has a lot in common with other messaging apps so it is usually quite straightforward for rebels to learn how to use it. However, in order to sign up to WhatsApp you need a smartphone, which not all rebels will have access to. In order to use WhatsApp on a computer, the smartphone that it's linked to must be connected to the internet continuously, which is a hindrance.
Features
You can't save messages for later or mark them as unread so you constantly lose key info and can't find it again. The message box is small and you can't thread effectively so it is difficult to keep track of a conversation.
Discoverability
Within WhatsApp, group admins can create and share links that rebels can follow to sign up to the group. This provides a quick and easy way to invite new people. However, unless this link is published somewhere for rebels to find, it is impossible to find the group to join in the conversation.
Other information
Another issue with WhatsApp is that many people use it to speak to friends and family. Having XR-related chats on the same platform can put unnecessary stress on rebels who might want to temporarily 'switch off' XR communications in order to avoid burnout.