The problem here is that the built-in password managers often have no password protection and this means that your sensitive data is not encrypted and is in plain text. (Safari does not have this issue.)
It is true that password managers that are built in browsers usually don't have password protection on it's own, but they are using password from login account on your computer.
So you can't easily access all details with password even if you are logged in to your account because you have to type OS login password to access this.
I wouldn't blindly trust this especially if you are using weak login password, and there are much better open source password managers like KeePass.