Tavis Ormandy called this, about Anubis, almost exactly a year ago:

https://news.ycombinator.com/item?id=44962529

It never really cohered as a solution. High-powered scrapers are better equipped to handle proof-of-work challenges than end users. Proof of work makes sense for a password hash, where any one guess at a password provides zero marginal utility. But every request from a scraper is productive to the scraper.