Earlier quoted context omitted.
Vanishingly few. Occasionally, I'm browsing the web and hit a text link that takes me to the browser's "I can't find this site" default screen. This usually happens with sponsored links that are not served from ad networks yet link to known ad sites.
My wife frequently complains about sponsored Google searches not resolving. She doesn't want to use an in-browser adblock, so the links will still appear, but aren't usable. Also, many redirecting analytics services from emails get blocked. Personally, I don't find these to be breaking issues for my use. My only issue is that the PiHole interface's administrative features are authenticated via the PiHole's service us…
Also to people redirecting ad servers to 0.0.0.0, that can cause page damage particularly with things like iframes. Pi-hole instead redirects them to its own webserver and serves up 1x1 pixel transparent images to avoid this.