This is Apple’s app and syncs beautifully across Apple devices.
There are two slight advantages of Keychain against smart lock:
The ability to generate passwords
It doesn’t only run in browsers
Keychain allows that once you connect your MacBook to a WiFi network you’re iPhone will automatically be able to connect too.
The major downside compared to Smart Lock is that Keychain is very much a personal system. There doesn’t currently seem to be a setup for it to be used as a business tool. This is where Smart Lock adds extra value.
Other than those bits, the two systems offer pretty similar services capably providing the necessary core elements of an effective password manager.
Like Smart Lock, Keychain is free and in-built for Apple users.