@isotopp You also could mention that Matrix protocol is the basis for "secure messengers" in the German Military (https://www.bwi.de/magazin/artikel/open-source-matrix-ist-einheitlicher-messenger-standard-fuer-die-bundeswehr), Public Services, Federal Schools (https://www.freie-messenger.de/messenger/verwaltung/) and Health Care System (https://fachportal.gematik.de/anwendungen/ti-messenger).
@oliof @isotopp afaik all of them build their own clients on top of the protocol. Hopefully, they spend some time on UX. But I doubt. Most probably, that are all themed Element Clients.