The problem with Manifest V3 is that it limits the number of built-in and user-added rules for extensions installed by an individual user. Since at the core of any ad blocker extension is the filtering rules that teach it to detect ads, such restrictions can become a real snag.
Mozilla's study finds that iOS and Safari privacy is not enhanced for end users when Private Click Management is enabled, and a small number of users can still be tracked across sites if the advertiser desires.