It's as secure as anything else out there. I have it set to not remember when the website ask it not to. So all financial related stuff doesn't save.
But for convenience it's great! Not having to enter your password and user names across the web is great. I use to use 1Password for this. Which was great, but it never could be integrated into the core system like iCloud Keychain can. On top of that, it's pricey. So for clearing website history and data. It makes it really easy to log back in. It's also nice that it sync's to all your iOS devices. So you can create an account on your Mac and when you go to it on your iPad, you can log right in.
It's fantastic for convenience, and it gives you some added security if you take advantage of it. Most of us for everyday things on the web have simple passwords for those accounts. You can now go and set up complex passwords for those. You can access the passwords in the Safari options menu and delete them.
What I'd like to see. For it to become more like an app. More like BB Password Manger.