Flask is a lightweight microframework for the Web. However, as happens with Django, one of the biggest drawbacks related to Flask and Flask-RESTful is that each HTTP request is blocking. Thus, whenever the Flask server receives an HTTP request, it doesn't start working on any other HTTP requests in the incoming queue until the server sends the response for the first HTTP request it received.
We used Flask to develop a RESTful Web Service. They key advantage of these kind of Web Services is that they are stateless, that is, they shouldn't keep a client state on any server. Our API is a good example of a stateless RESTful Web Service with Flask and Flask RESTful. Thus, we can make the API run on as many servers as necessary to achieve our scalability goals. Obviously, we must take into account that we can easily transform the database server in our scalability bottleneck.