I can't really comment on the financial/entrepeneur side of Pinterest, but on the user side:
* It's a walled garden, closed source, force signup, the UX is bad even if aren't a user (dominating Google Image Search taking you too pictures you can't click), it has lot's of dark patterns
* It heavily suggests signing up with Facebook or Google, making all the critic on privacy of those platforms also theirs
* It has all the features for the mob-mentality, abuse and so on (Like system, sharing, popularity based rankings)
There aren't as many of these problems visible because of the content(no sensitive fotos, banking information or much politics) but that's not because of their great buiseness model but the theme of the platform...