I've worked with small to medium sized 3rd party apps in a variety of industries and although they're all easy to access their data (common RDBMS products) and/or export it to another system, they do a poor job of providing any type of customizable import or API. This is find for reporting, but you end up with redundant data entry in multiple system with the risk of poor data integrity. It's difficult to make those features available and for vendors who offer multiple "modules" they may prefer you stick with their entire system instead getting your functionality elsewhere.