Hardly an argument. The clients warn against it,it’s not default, has to be done very intentionally,etc. That’s on one level with “I screenshot messages and post them on facebook” - and in that case the other end would not be warned about it.

The downsides of Matrix exist (key exchange, performance, audio/video, federation issues), but this is not one of them. Overall it’s as secure as Signal but, I totally must agree on that, not as easy as signal. Which partially comes with the federation, but partially also due to ongoing issues the current development fails to adress so far.

Threema has the downside of being swiss hosted (which sadly is an issue by now due to their legislation) and being from a company very close to the swiss goverment but at least the Orange Tittler can’t shut it down directly.