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Being built from two of the oldest Node libraries where each of them has had several releases and constant bug fixes and patches means that express.io is a rather mature solution for building realtime web applications.
Pro Large userbase
Express is the most used Node.js web framework and this brings a large number of learning resources, guides, tutorials and many libraries available for it.
Pro Can easily be upgraded from an Expressjs app
Since Express.io is basically just the Express framework + socket.io, it's very easy to upgrade an express app to an express.io app.
npm install express.io
require('express.io') and it will run the same as before.
Con Working with relational databases can be a hassle
Not only express, but every Node framework has this problem. While there exist plugins and libraries that allow developers to use relational databases in their web applications, they are not very good and usually it's a hassle and more trouble than it's worth to use a relational database with Node and express.