The biggest problem that I have is that it's privacy built on a closed source platform that works extremely hard to wall users in and create substantial vendor lock-in.
As someone who has tried multiple times to dip their toes into the Apple world, I can tell you that it's highly hostile to users who try to pick and choose how much they want to be an Apple citizen. If you don't agree to sign up for Apple cloud on the latest iOS, it feels like 80% of the basic functions for a smartphone get locked away from you.
And they have viral aspects to their lock-in as well. Have a single family member who isn't on iMessage? Looks like they don't get to participate in your family discussions anymore.
Their privacy features are a selling point today, but your privacy is 100% in their control. If the executive team made the decision tomorrow to switch Apple into a fully panopticon ecosystem, they'd be able to switch over in a year with little visibility for their users that this transition was happening.
If you are in the Apple ecosystem, you under heavy lock-in pressure and you are fully putting your faith in Apple that the directions they go in the future will be directions that you are happy with.
That doesn't feel secure to me.