It is basically WhatsApp 2, with a source-available client. The client isn't free unless forked and used with some other server, because Signal can ban you solely for using a fork. Not being federated may have minor security benefits, but there is no excuse for not having the client accept connecting to multiple servers; they just want to lock you in. Normal people should use Matrix, or XMPP. I am much more concerned about Signal paywalling things and locking me in and so on than about someone knowing at what time I send messages to someone.