Its a great messenger UI sucks but it works for the most part.
Main problem i run into is not receiving messages until i open the app
Discussion
That’s death to a messenger. Same problem on simplex.
The Reason Behind Delayed Notifications on Certain Android Devices
https://threema.ch/en/blog/posts/push-android
Solution
https://threema.ch/en/faq/nopush
or
or Threema-Forum.de