Is it secure ? a. From Apple as a firm ? b. From also having just one gateway to all your user and password details at one place if your iCloud details get compromised either through formal channels such as govt, courts or informal channels like hackers.
Everything stored in your iCloud Keychain is secure. (a) iCloud Keychain is protected by industry-standard encryption. Any other Apple device that tries to set up your own Keychain will not be successful unless you approve their access. (b) Since iCloud Keychain is used for remembering passwords on webpages and it stores your details there, it automatically displays a username and fills the password in for you. Your passwords will be securely stored and synced to your Apple devices. There are, however, apps and viruses that can be used to hack your iCloud Keychain so make sure to use devices that you trust and that you have antivirus installed.