No, the goal of these security products is to defend against the government, not a random guy. In that context, it's extremely important that their server undergo the same level of cryptanalysis.
> the server can perform a MITM attack. > you cannot detect MITM between you and your peers.
>> NOT true. You can compare key visualization in the clients.
The key is not shown in hex, so a MITM is quite simple.
When they do enabled it for the first time, we can instantly MITM them using the attack against the "image verification" I mentioned lower down (https://news.ycombinator.com/item?id=6932053), and we can assume that the conversation is worth our while listening in on. The user will hopefully expose themselves in the belief that they are safe, and the game is over.