Before you go shaming those bad, bad Chinese companies, imagine my surprise when I restored my Apple iPhone from iCloud only to find my (always set to private!) Safari back there with every tab I ever opened.
Only the things under "End-to-end encrypted data" are actually encrypted in a meaningful way - everything else just sits on servers that use disk encryption, but that fails to defend from 99% of realistic attack scenarios.
A lot of content - including photos (which I used to use to take pictures of sensitive personal data, like ID cards) - is not meaningfully encrypted.
Thankfully, Safari tab data is actually encrypted as of iOS 13.
While Apple has done a good job of protecting user privacy compared to the alternatives, they still have a lot of work to do before we can accurately say things like "iCloud backups are encrypted".