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.)

The white paper(s) are regularly updated, and were last published Spring 2020, so "a single point in time" is not accurate.

https://support.apple.com/guide/security/welcome/web

I know this is not really scientific, but: Apple's business model makes me more likely to trust them over Facebook.

Same. I'm more inclined to trust apple on privacy than most of the other big companies.