Threema is coming to F-Droid

I know no one IRL using either of those, but many on threema in Germany

(If I would guess at the meaning behind your question given the examples mentioned, the short answer would be: I'm not so fanatical about privacy that I prefer complex glitchy software with missing features over software that has proper encryption but uses a central set of servers.)

Element is a world of UI glitches. At least encrypted chats are functional now (until ~last year, decryption errors made it close to useless without a fallback channel (e.g. Signal) for the person you're trying to chat with). Still, no search in the Android or web client (only on iOS for some reason)....that is, unless you just don't turn on encryption! So it's not a serious option for me. I use it with one person, who has it on iOS and so occasionally gets requests from me "hey what was this thing again we talked about regarding xyz". We also both have Signal to fall back to on occasion. I don't remember if calls work or if those are encrypted nowadays, weren't they partnering with Jitsi here, and Jitsi's e2ee is still in the works last I heard/checked?

Briar is this Tor or mesh thing right? That's very feature-limited since Tor cannot reasonably support calls (the experience would be like trading voicemails). Mesh is very, very cool tech (I love self-supporting things) but there are no nerds close enough (that I know of) that I could actually talk with them practically using the mesh, even if I was willing to break the law on transmission power (and both phones would support that... like, even theoretically I can't generally use this). I think I did try Briar because sometimes you want to chat with someone in wifi range but outside of network coverage. Since I don't have Briar installed, even that must not have worked between me and my girlfriend's phone. I vaguely recall some device discovery issues but I've tried all the available mesh apps on f-droid so I may be mixing things up.

Threema is pretty crappy as well, but by the Telegram standard I've been used to (back when I believed the missing encryption was just growing pains and they'd get around to it any day now), there's nothing remotely close to feature parity anyway. Threema does fail at basic usage for me, though, which is any kind of functional notifications and a functional desktop client.

Yet I think it's important to have more than one option out there (the "one option" around me is facebook's whatsapp) and I'm happy to accommodate anyone that uses some open source chat system. I'm less okay with people that request I install some proprietary messaging solution just to talk with them. (If we wanted to chat and you've got Briar, sure yes I'm totally installing that, even if it has issues that make me not use it as my primary system.) I've got Threema out of curiosity, initially I wanted to see if I could avoid Signal's connection bugs and Wire's glitches, though today I only still use it for one contact whose Signal does not receive my messages at all.

What Threema has going for it, is that it's not using USA infrastructure, which is quite unique for centralized chat software (for better or worse, at least now you have a choice), and nearly as stable as Telegram: things generally just work as designed (major exception being notifications). Features are fairly extensive so that would almost make it a serious option for my primary messaging system, though the two missing essentials are what kill it for me.

The only usable systems, when I last checked ~2 years ago, were Wire and Signal, each with unique but surmountable downsides (Wire is sluggish and more glitchy and much less of a network effect; Signal misses features like message editing and silently suffers regular connection issues). Everything else I know of is not usable for my family (bad UX and/or glitches that'll throw them off), not encrypted (Telegram), missing features (e.g. video calls, search, desktop client), or unusable due to instability (Matrix until recently, though still not a great experience on my friend's home server).