Some very knowledgable iOS security people have told me how hard it is to break iOS. You need quite a few chained exploits to do anything meaningful. Browsers are pretty much the only things with Read/Write/Execute memory.
Security is always a convenience/security trade-off. iOS is about as good as you can get before inconvenience will turn people to less secure devices.
There is the question of vanilla iOS being susceptible to attack from NSA and any other malicious attacker and then there is the question of is the security design sufficient such that even apple could not backdoor your privacy should they desire. The latter is what the original commented was addressing. I think the statements from apple do not clarify this matter and we should assume they can backdoor should be required by warrant. I need to see detailed technical specifications for their claimed security measures before I would trust these statements (if someone has a RTFM link, much appreciated).
Security is always a convenience/security trade-off. iOS is about as good as you can get before inconvenience will turn people to less secure devices.