[shalmanese]

Right, but that's not software that runs only on the client. There's still a server sitting in the middle.

iMessage theoretically also does this. The difference is how key exchange is handled, with iMessage preferring a more usable but potentially less secure approach.

[Natanael_L]

The real problem with iMessage is that public keys are unverifiable. The apparent lack of PFS is also a negative. And the iCloud backups in plaintext (although optional).

Signal have none of those particular problems.