edit: Thank you for comment!
I think, the fact that it is decentralized, your data is not in control by some company. How would you make sure that whatsapp servers are secure?
Immutability, you can pretty much make contracts and refer back to it in case of disputes. As I mentioned there is an argument can be made, if it is immutable, if data is stored on blockchain forever then How do you make sure that encryption will not be broken in future? I think if we have confidential transactions that would solve this issue as well.
Encryption itself probably will be similar, with multiple choices.