I kinda forced a friend of mine into using element/riot. He mostly complained about the lack of "polish" (which I also feel, and I'm a rather "frugal" user). He proposed Threema as an alternative, so I guess that the polish is quite the missing box.

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?