Apple is unable to access the data even if subpoenaed, for example, and this is provable via binary audits and client verification that they are communicating with an auditable node.
I don't see that apple software is any different in that regard (just try using Mac OS for any length of time even on apple silicon and you run out of fingers to count obvious UI bugs pretty quickly just in day to day usage). And obviously AWS won't be able to decrypt your data without your keys either.
The people running these huge multi-multi-billion clouds are not idiots making fundamental errors in security. This is why they all pay mega salaries for highly skilled people and offer five-figure bug bounties etc - they take this seriously. Would some random VPS or whatever be more likely to make errors like this, sure - but they are not in (and not expected to be) in the same league.
I would not trust Apple any more or less than other other big-time cloud provider.