There's a whitepaper on iMessage's security as it was at a single point in time. It may be possible to verify this on a recent build with enough effort, but the average user will never be able to make that validation. So we can't be completely secure there.

More secure than WeChat? ABSA-FREAKING-LUTELY. And do I trust Apple to remain secure? Yes.

I'm mostly putting this here because I've heard people talking about security guarantees without considering tweaks in the supply chain or binary deliveries. Even Signal could get breached with enough effort from Google to push new bits and bypass certificate validation. (Though practically that is not going to happen.)