It does block Youtube ads but very poorly. Sometimes the ad plays but a refresh immediately removes it, other times it can take multiple refreshes. It's not perfect like it is when i use Firefox or Opera so I stick to Yputube on those and Safari for general browsing.

Sidenote: Also Gmail is insaaaaaaaaaanely slow on Safari and a lot of sites just don't format well or ad blockers completely break the site. Safari is by far the worst browser on Mac for site compatibility and also extension selection but I use it for better battery life when not plugged in.