It's massively less annoying than a captcha, which is both a longer delay (typically, at present) and a massive cognitive distraction/roadblock.
The anubis author has stated they recognize it's an arms race, but PoW scales. Captchas and other signals are already at the end of the road; any additional difficulty increases false bot-positives, which are already unacceptably high.
For websites running dynamic languages, a binary (anubis is in go) sentry that operates before[1] the website is forced to expend any resources, is usually a large improvement over a site-hosted captcha. I would rather, and I think most humans would agree, have to wait a few seconds, maybe even closer to a minute in the future, to get a website access token good for a day or a week, than be forced to solve a captcha.
The dilemma for bots: when tokens are bound to the connecting ip, scrapers must limit the connecting IP pool for each site they want to scrape, becoming much more obvious and easy to block, or they have to use massive amounts of compute.
[1] this is true regardless of whether anubis is in reverse proxy mode or auth mode.