For me, the only impact I've ever seen is trying to view embedded images in some web-client emails (outlook.com in particular, but not all). In these cases, the images would not load unless I used the BLOCK_ICLOUD_PR=false line in pihole-FTL.conf. There may be other ways around the specific case I encountered, but that was my observation.
For what it's worth, I've reverted to the default Pihole behavior of blocking the Private Relay function, and the devices themselves are set to use Private Relay when they can (i.e. when Pihole is not in use).