Briar shares your hardware's Bluetooth MAC address with your contacts even if you haven't added that contact or connected with that contact using Bluetooth. meaning all your briar contacts even those on groups are storing your MAC address ( a unique device identifier) which makes plausible deniability virtual impossible. I remember reading the devs themselves acknowledge that briar isn't a anonymous chat app (somewhere in their git repo or the wiki or the blog, can't remember where exactly), so beware of this aspect.

This is a myth that's been addressed by the project. For starters, there are no disclosures about the amount of nodes owned by the Government/NSA/CIA/etc. You're probably thinking how the project, in 2012, received 80% of its funding from the US Gov source.

You may make the argument of "follow the money", or you could also make the argument that this type of tech this widely distributed benefits the government too (field agents for some agencies allegedly use Tor, as do foreign defectors) and compromising the network would lead to a potential vector to compromise their interests.

It's also worth noting that Tor uses 3 hops (entry, relay, and exit nodes) and you can check the location/IP of your current route at any time to ensure geographic diversity. An actor would have to own all 3 to know what you were visiting and trace the traffic back to you.