It's more a question of "what does does Element do that Threema doesn't"? Element is a Matrix client, so you connect to the global open Matrix decentralised ne

It's more a question of "what does does Element do that Threema doesn't"? Element is a Matrix client, so you connect to the global open Matrix decentralised network, rather than just closed-source servers run by a (newly-sold) company like Threema. Typical Matrix implementations are entirely Apache licensed open source, so you get full control and visibility both clientside and serverside. Matrix also already provides synchronised multi-device conversations, as well as more advanced cross-signing verification for E2EE key validation than Threema. Finally, Matrix is an open standard protocol maintained by the non-profit Matrix.org Foundation, that anyone can contribute to via the open governance process (https://matrix.org/foundation https://matrix.org/foundation, https://matrix.org/docs/spec/proposals https://matrix.org/docs/spec/proposals etc) - the Matrix standard gives developers full freedom to write their own clients/servers/bots/bridges/whatever. Personally I'd say that Element & Threema have similar levels of UX polish these days (but I'm biased, working on Element). It's perhaps true that Threema's relatively limited featureset means that polish is easier to achieve.