They're very hard problems, but still, less hard than blocking AI with CAPTCHAs.

[citation needed]?

After all, Anubis looks to be a successful project to me.

Anubis doesn't block anything. You just have to open the page and wait a few seconds before you can see it. It's just that current crawlers are too dumb to do the proof-of-work, so it blocks them until they can do it.

How exactly does this demonstrate that it isn't successful at blocking them?