When comparing Messenger vs Unison, the Slant community recommends Unison for most people. In the question“What is the best team chat software?” Unison is ranked 70th while Messenger is ranked 74th. The most important reason people chose Unison is:
It's possible to @user or @all to alert a specific user or everyone in a room, respectively.
Specs
Ranked in these QuestionsQuestion Ranking
Pros

Pro Uses Facebook friends to create a contacts list
Allows communicating with anyone who's a friend on Facebook.
Pro Read receipts
Can see who has seen each message in group messages.
Pro Easy to share posts from Facebook
Integration with Facebook makes it easy to share any interesting posts from the Facebook timeline.
Pro Great UI
User interface of the application is clean, intuitive and easy to overview.
Pro Notifies via mentions
It's possible to @user or @all to alert a specific user or everyone in a room, respectively.
Pro Full end-to-end encryption
Pro Native apps for iOS and Android
In addition to being available on the web, Unison has native iOS and Android applications.
Pro Shows who has read each post
Pro Built-in support for video conferencing
Video calls are a single click away.
Cons
Con Lack of privacy
Messenger is owned by Facebook. Based on their privacy policy, this means that any information you provide (including messages sent) are used by Facebook to profile you and sell access to your data to advertisers.
Con Lack of end-to-end encryption
Secret (end-to-end encrypted) chats are only supported in one-to-one messages, and are not enabled by default, and the encryption has not been verified independently.
Con Lacks wehbooks & services integration
