Believe it or not, I don't use mobile phones so I have no direct experience or insight. But... I was under the impression that smart phones were generally less secure than the average Linux desktop. Am I completely misinformed about this?
Depends on what you mean by "less secure" really. I do almost everything I would do on my desktop, on my smartphone. Never had any sort of security issue. You may want to be more specific.
I was just having a brief chat with someone about the merits of Jaxx and Coinomi and I pointed out that, despite its plaintext seed issue, at least Jaxx has desktop version and isn't limited to mobile like Coinomi. The other guy regarded mobile as more secure than desktop due to desktop being a bigger, easier target for attack. I'm not so sure about that but I don't know anything about mobile...
iOS has a pretty good isolation model (and defaults to encrypted storage) which means that the plaintext seed is more secure on the phone than on most Windows desktops.
Whether a filesystem is encrypted or not, if the intended user can read it, what's stopping any other sufficiently privileged process or anyone else that has improperly gained access?
I believe other apps are precluded from accessing the wallet's data by iOS's app isolation. Obviously, physical access to the device and use of a weak passcode or Touch ID would render the encryption useless.