Computerworld - https://www.computerworld.com/article/3276659/mobile-wireless/how-to-use-apple-s-messages-in-icloud-for-ios-mac.html#tk.rss_all
Along with key HomePod improvements, Apple also introduced Messages in iCloud with iOS 11.4. It’s a useful feature designed to store your Messages and attachments in iCloud, but enterprise users should think twice before enabling it.
Security is everything
I’m not saying iCloud is not secure – so long as you use a six-or more digit passcode or (better, but more awkward) an alphanumeric passcode, it’s highly secure. I’m reasonably confident a strong password, Apple’s own systems and its insistence you use two-factor authentication is enough for most of us.