Are such third parties listed? Can you inspect their reports? What testing methodologies are involved in order to issue such certifications? And can we see such certifications at all?
I can't think of any entity I would trust with securing truly sensitive information. For important stuff, do it yourself. For simple things, including bank accounts and such, I see no issue with trusting Apple.
I don’t know I buy the “for truly sensitive stuff do it yourself” line. That’s like saying for the truly lethal substances handle them yourself. Most people aren’t more skilled than the apple security folks. You’re almost certainly going to screw up your encryption or leave some vulnerability unpatched or unknown. Frankly I consider my iOS devices to be some of the most secure systems I have access to, and reading through their security documentation has informed that opinion.
The cynical view, of course, is that Apple's incentive and the Third Party's incentive can become very much aligned for the right amount of money.