SSB is basically a database log that syncs with your peers. Your log is immutable and message order is preserved - every message+previous_hash is signed - a hashchain that enforces order. Your peers can verify that the log has not been tampered with (both order and contents).

This doesn't work with multiple devices, since they'll be two different logs. However, there is ongoing work to support multiple identities. One identity could broadcast that another identity is a "sameAs" - causing friendly peers to sync with the other identity as well.

Personally, I haven't found this to be a problem. I find separate identities less distracting. (My favorite reason to use SSB is that continuous syncing is not an expectation.)