The system is very flexible, and it even has some build in functionality for web services. You can connect to their service using WSDL and SOAP or by using HTTP requests to get JSON, XML, CSV responses back. I think there may also be PDF responses? I'm not sure since I haven't used them. Here is one of the big issues: I said you could use HTTP requests, but you don't really get to make RESTful requests. You end up with these horrendous lists of parameters on your requests that can get very unruly.
That is really it for big issues. They have taken a lot of the annoying aspects out of making a web application. They take care of rendering for the most part, they have ACLs, they have logging, they have scheduled tasks. They even have decent documentation, which is always nice to find.