Isn't the problem that they could be compromised enough already? Here's some thing that could eventually lead to issues:

1. They have your phonenumber. Which in 99% of the world means it will lead to you when things get "seized" (stolen).

2. Their servers are centralised meaning that if things get "seized" (stolen) they may at some point find a hole, or a a brute force attack vector that will be able to do something with that data.

I am not saying it is not secure (right now) or you should not use it, but it makes it really easy to apply a lot of pressure on a single point and that is something you have to consider if you need absolute privacy.