Briar is good, it has just has a different positioning.
MTC is a balance between standard rich multimedia real-time messaging, including audio/video calls, and privacy (full peer-to-peer).
Briar's design, based on Tor, limits the possibility of a full messaging experience (WhatsApp-like), but it's strong on metadata hiding, and its target users are different (activists and journalists in hostile or censored environments, etc.)
MTC's target users would be standard messaging app users with some more attention and concern about protecting their private conversations, without giving up all the standard messaging features they're used to.
It's mostly about positioning.
MTC aims for a balance between standard rich-media real-time messaging, including audio/video calls (WhatsApp-like), and privacy (full peer-to-peer, no registration, no phone number).
The target is a standard messaging-app user who wants more privacy for their conversations without giving up the features they're used to.
Jami uses a very similar set of protocols, the main difference is how peers are discovered, Jami uses a distributed hash table (OpenDHT) where every device is a node on the network, which can mean more setup friction and a more technical experience, aimed at a more tech-savvy audience. One side effect is that your IP is visible to DHT nodes, in MTC it's only ever exposed to your actual contact and the TURN relay.