I built couple of websites in the past month using MEAN.js and love it for the following reasons:
1) They selected a pretty good and widely used set of technologies like NodeJS, MongoDB database with Mongoose, Express, Angular, Jasmine, Grunt, Passport.
2) The framework code is very well laid out with functionalities segregated into their own modules.
3) It includes good support for unit tests for both the apis and front end. Some tests get generated as part of the boiler plate code when you install the codebase and you can build from there.
4) They use Yeoman generators to create boiler plate code for any new functionality that you want to add. This is the reason I preferred MEAN.js over MEAN.io. The generators enabled me to be productive very fast. You can start with the boiler plate code that creates the basic CRUD for your feature, and you can build from there.
5) The basic authentication with sign in and signup already comes with the framework including integration with social logins. I have spent considerable time at the start of a lot of projects implementing just that.
6) It is very easy to run and deploy.
7) They have a good community on Facebook where you can share and ask questions.
So it proved very valuable for me when creating small prototypes or simple websites in a very short time. I will choose it again for creating any new prototypes that have similar use cases. There are obviously a lot of other choices which might even provide the above 7 features in some way or the other, but MEAN.js came out to be the winner when I did my research couple of months back.
Having said that, it would be naive to think that it can be the solution to every problem. When creating large complex scalable solutions that will exist for long time, each and every choice of technologies have to be well researched and chosen with care. For e.g. a non relational database like MongoDB might not be the right solution when you really need a relational database. NodeJs is great and very popular for it's non blocking code but traditional languages should be preferred for very complex and heavy CPU intense processes. AngularJS fits the bill for the most part, but you might prefer React if a few microseconds difference in rendering speed is still important to you.
In conclusion, using a framework with a set of technologies and boiler plate code works very well for rapid prototyping and generic solutions. And MEAN.js does well in that respect. But any framework like that will start to become less useful when each technology decision is mission critical for your app.