As an Amazon Associate I earn from qualifying purchases @AMAZON
MPG has not evaluated Apple’s iCloud Keychain. The appeal of a 3rd-party product like 1Password is a general approach covering all the bases, with the developer incentivized to add value beyond more limited Apple functionality.
Readers are encouraged to make their own evaluation.
The MPG perspective is that of a professional computing user with mission-critical data and workflow, not ad-hoc entertainment usage and similar (music, videos, casual photos, etc). What follows is a personal view developed over time by too many negative experiences with Apple software releases (only a smattering given here)—users are encouraged to see MPG’s viewpoint only as one data point, and to find their own comfort zone.
In the past few years, MPG has suffered far too many issues with iCloud, including destruction of bookmarks and contacts by iCloud syncing and baffling syncing messages across multiple computers.
Apple Core Rot and various Apple security lapses and an increasing disavowal of professional users over time have also left a strong distaste for relying on Apple for critical needs (OS X excepted in spite of reservations).
Then there is the trust issue: Apple arbitrarily changes technology over time, and generally does not tailor services to anything but Apple-centric services, and increasingly only towards iPhone/iPad/iPlay.
For all these reasons, MPG prefers to use a password manager developed independently, since such programs must excel to compete against stock offerings from Apple. As of late 2014, 1Password is that program.