Probably nothing; Apple talks a good privacy talk and I always recommend Apple products to my non-nerd friends and family because of it.

HOWEVER, I can't really get past the fact that it's closed source. At the end of the day, that means that Apple can say they do XYZ practice, and it's very likely to be true (some is testable, and imagine the shit-storm if a whistle blower showed that they really do store your keys, etc), but what if it does something else sketchy "instead" of the things they promise they aren't doing?

Pretty much, if I care about privacy, closed source anything never makes it on my list.

Nothing, as far as I can tell. End-to-end encrypted history and tabs, and recent versions with iCloud+ use encrypted DNS and VPN (iCloud private relay) that uses encrypted forwarding so ingress servers can’t tie your requests to a destination.

Outside of using Tor and your own VPN, it looks like Safari w/ iCloud+ protects browsing history from everyone (Apple and ISPs) while delivering a seamless experience.