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/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.
The box I'm missing in Threema (unless I'm wrong of course) is the "not centralized" one. If for some reason (and admittedly, there aren't many realistic ones), Threema's servers/organisation are corrupted, the whole networks falls, right?
There are still major, glaring and game-breaking bugs and UX quirks in Element even after a full frontend rewrite (that, by the way, made security UX significantly worse).
(this is a US-centric comment, as phone numbers are rather static here; unlike other countries, not having a number isn't really how it works for most people on the big 4 carriers and their secondary MVNOs)
48hour refunds on google play should have addressed this conundrum but it seems people just aren't aware of it :(
But there's not a single non-German contact on there.