Poor customer service, features that don't work, and even basic functionality like sending responsive e-mails are lacking. You lose the ability to do basic e-mail campaigning with this solutions.
Can think of none except that they do an excellent job of selling you something. What you get is a totally different story.
If you're looking to sending dynamic, targeted marketing e-mails, set up some basic triggered events for marketing communications and gain some small amount of automation...STAY AWAY FROM BOOMTRAIN.