This is an interesting article. While the author isn't necessarily claiming signal is bad, they certainly could be leaking everything or providing backdoors and we wouldn't know...

I wonder what the downside of a decentralized chat service like matrix is in terms of UX?

You have to host your own instance (probably hard.. ) or trust the person/org of the instance you are using which is kinda the same as using signal?

That you don't have the best UX for sharing contacts?