Assuming it's actually end-to-end encrypted, it is susceptible to MITM attacks because you're trusting a centralized source with key exchange and verification.

You cannot perform an audit on your own. You are trusting that you received the correct keys without a way to verify identities outside of the network.

That is not secure. Please read http://blog.quarkslab.com/imessage-privacy.html

>something engineering designed to be end-to-end encrypted?

Is that seriously what you believe?