How secure is FaceTime by Apple

If you believe Apple, FaceTime traffic is end-to-end encrypted using AES-256. This is secure, in the sense that somebody intercepting the traffic can not decrypt it (as far as is publicly known).

However, crypto is hard and the security could be compromised if Apple has made an implementation error. I.e. because of a programming bug the traffic may not get encrypted correctly. Apple has previously shown that they made pretty secure systems (e.g. the iPhone in the FBI case), so this is not likely.

Even if the traffic is encrypted, the system is not secure without authentication. If you set up a connection to the wrong person, an encrypted connection is not going to help. In that case you may tell your secrets to somebody pretending to be someone else. Of course in the case of FaceTime it would be pretty hard to impersonate someone, because you can see their face and hear their voice. Verifying that you are talking to the right person is pretty easy, even though there is no technical solution for this in FaceTime.