I agree Briar always has seemed even more decentralized and security conscious than Matrix. I tried to use Briar with my family for awhile and it worked well but its always on function was a bit difficult on mobile (it seems like something that would be worthwhile for some circumstances but maybe not worth the cost for just chatting about groceries).

However, in reading the article, they discussed group communication functions in Matrix, which were absent from Briar for awhile.

They also mention that Matrix is planing on introducing mesh networking, which is very similar to Briar. I found this extremely interesting. It seems like Briar and Matrix are sort of coming full circle to each other. I'd love to see either or both take off.

There's also another obscure and less well known messaging app called CWTCH https://cwtch.im/ It's still in development though...