The work they did behind the scenes was impressive in the 1990s. Today it's only impressive because Craigslist is built with antique technology.

Scaling mostly-static content with zero page customization is not rocket science anymore. Just ask anyone in the porn biz.

Scaling is not the issue. Keeping it relatively clean is the issue. Dealing with criminals and law enforcement. Keeping the community satisfied. Classifieds ads sites are a magnet for spam and all sorts of nastiness, and Craigslist is the biggest one.

By this evaluation, they have utterly failed. Craigslist is full of spam and criminals and users who are bitterly unhappy with the lousy UX.