EmailLabs/Lyris decided to invent their own XML-over-HTTP API format rather than use REST or even SOAP or anything else you might already have a library for. They combined this with a very poor understanding of how XML is supposed to work (lots of <DATA> tags) and return results that aren't guaranteed to validate. Dates are represented in one of six different formats depending on which particular function you're calling. If there's a problem, you might get an error, you might get back nothing. I could go on.

Silverpop is similar.