For our use-case Persona is great. I've personally built the "web account" solution several times for different clients and I did not want to build it again. Persona is the lightest drop-in and easiest to implement solution you can imagine, and it works extremely well.
We've had a couple of issues, but nothing of significance. Mostly these have been us doing things unexpected, i.e. pre-loading user accounts obtained face-to-face and the case of the user@ part of the email address differing from the Persona provided email address.
I personally also wish that persona.js is added to CDNJS to increase the speed by which it's served.
What we found through asking, was that people feel very protective of their Google, Twitter, and Facebook accounts. They will sign in with them, but there's a wariness of doing so as they do not want to be spammed, have things posted on their behalf, etc. These fears mostly apply when people first arrive on one of our sites. We wanted a lower friction to that initial sign-in, and we feel that Persona gives us this.
We also found that on interest-based communities there is a reticence to associate real-identity to the nerdiest of their interests. This meant that not using Facebook and Google is a good thing in our scenario.
One of the things we like about Persona is the user experience. For our product, simplicity and ease are core goals. Persona helps rather than hinders on this front.
We really like Persona and hope that it remains under active development for a long-time.
We are not of the belief that it's failed, simply that it's a slow-burner and needs some marketing support.