The main problem with iMessage is the limited audits. It's a very complex application and with that complexity comes higher risk for vulnerabilities to slip through. It'd be way less of an issue if it was at least source available so more people could audit it, instead of just whomever gets permission from Apple.
Like Apple's bounty system is great and all, but it's not as effective as for example being able to directly scrutinize signals open source codebase.
The problem with Apple's approach is that even if someone finds and reports the vulnerability for the bounty money, that still means that vulnerability has made it into the production code and thus made it onto consumer devices. Unlike an open codebase where the vulnerability is more likely to be caught before it reaches consumer devices.

You can argue that the code being openly available isn't necessary indictive of being secure, which is true, but it's certainly more favorable and for signal it's worked out very well. Also there's plenty of state sponsored hacking groups trying to break signal. Maybe not as many, sure. However it's a lot less likely they'll succeed in breaking signal then iMessage.
Infact, just recently signal had to dismiss a 0-day vulnerability report people were freaking out about because it turned out completely bogus. Thought, I suppose that's beside the point.

"Security through obscurity is not security" - some guy idk.