What is the disadvantage of building point of sale based on web service?

I am going to build a Point Of Sale Web Application. This point of sale consists of client web application and PHP web service at a remote server. The client web application will call the web service using Ajax (JSON format) inside a company network. The point of sale will be used on computers and tablets.

So, do you know the disadvantage of building Point Of Sale web application based on web service?