Apple secure notes. How secure?
From a cryptographic standpoint (ignoring implementation issues and password sniffing), the security depends primarily on the entropy $s$ in the password, and the number $n$ of iterations in PBKDF2. Work to break is about $n\,2^{s-1}$ SHA-256 (for large $n$).

Per the obligatory XKCD, a good password has like 44 bits of entropy; a bad password (among the 1000 most common) like 10 bits.

According to that source, $n$ was $10^4$ in iOS 4.x; I'll use $n=2^{16}$ (6.5 times more).

Thus the effort to break the system is like $2^{25}$ to $2^{60}$ SHA-256.

$2^{25}$ SHA-256 is very easy. $2^{60}$ is considerable, but feasible for an entity like the NSA if it decides to invest in the necessary hardware (ASICs specialized in cracking PBKDF2). As a comparison, according to this source, bitcoin miners currently hash at a rate of $10^{19}$ SHA-256d per second, where one SHA-256d is two SHA-256; that is over $2^{70}$ SHA-256 per minute.

Nowadays, if something is protected only by a password, and can be accessed on a mobile device, and the key stretching step uses PBKDF2 without the hash in hardware, and the password is of average quality, likely it could be broken. For something safer, we want scrypt or Argon2, that leverage RAM to make the investment in a password cracker much higher; or an exceptionally good password.