* sub-par user experience: WhatsApp is just nicer and smoother, and people tend to like that
* very few people understand that Signal DOES NOT get your full contact list, while Facebook (through WhatsApp) does
Especially the second point is very relevant with the current situation — you do not necessarily want to expose your entire social graph to Facebook. But so few people understand this, and even fewer grasp that Signal can still work without doing the same thing.
The full contact list is uploaded to Signal servers by the phones. The only protection layer that users have is the questionable security of Intel's SGX.
It's still much better than what WhatsApp is doing, just not a black and white situation.
To add a point to your list: Signal does not have automatic cloud backup of messages, unlike WhatsApp. On WhatsApp, 30% of users have cloud backups enabled [1], meaning that you can basically assume that any reasonably sized group's messages can be accessed by people who have subpoena-power over Google (chance that there is no backup-enabled account in a group of n people is (1-0.3)^n... for 6 people it's already 12%).
I say theoretically because these schemes all have a core problem when they're not federated - you have no idea what your client is really doing and it's the client performing remote attestation with the enclave. You have no control over it. It could update tomorrow and switch every last bit of encryption off. Or it could do RA but not pin the enclave hash to anything audited (i.e. it accepts any enclave signed by Signal).
It's not a theoretical problem. Facebook say that WhatsApp is end to end encrypted, in the same way as Signal. That didn't stop them blocking people from forwarding links related to coronavirus. The literal and entire point of E2E cryptography is to stop them monitoring and interfering with people's communications, Facebook have been assuring governments for years they're powerless to do that, but of course the moment Facebook wanted to fight "misinformation" it all went out the window.
Fundamentally Signal and WhatsApp can never provide meaningful encryption or privacy. They don't allow alternative clients, so regardless of how much code they throw into the mix they control the entire pipe end to end and can just as easily switch it off again. And the moment their employees feel they have a sufficiently good motivation, it'll happen again.
Source?
Pick any version of the story. Or read their blog post:
https://blog.whatsapp.com/Keeping-WhatsApp-Personal-and-Priv...
How do they know a message is forwarded? The encryption is meant to make identical plaintexts encrypt to different ciphertexts, so obviously they must be leaking the forwarding status in unencrypted parts of the message. And why is an encrypted service trying to combat misinformation to start with - isn't that a contradiction in terms? These things raise difficult questions. You'd hope that once a service decides to go fully encrypted, its staff would believe that what kind of information going over it or how accurate that is, isn't any longer their concern.