My personal experience with Threema seems to differ from the general view on it: We use Threema for private communication within my team at work. I am the group admin. When my iPhone broke two years ago I had to do a full backup, because there is no other way on iOS to move to another phone with Threema. This has not changed since then, because I had the same issue when I upgraded to a new phone 2 months ago. It was not even possible to assign an additional admin nor to pass my permissions. A colleague had to clone the group (at least there is a function for that). Of course I still get push notifications for the old group- even though there is no way to join it from my new phone. The only thing that I like about it is the integrated poll feature.