Unfortunately, I can't say much about the contents of the DLC, as I had to disable it pretty early into my first run with it. It introduces the Slave Market, which has a feature of notifying you about one of your original pops showing up for purchase. This notification cannot be disabled like some others, which becomes a problem if your species gets traded around a bit. Trying it on a late-game save, I was getting a notification every single tick until it fills up the entire notification bar. Stellaris always had an issue with excessive messaging that can't be disabled with Shift+RightClick, but this is just a bit too much for me. There are mods that disable some of the notifications, but I've yet to find one for this particular culprit. A supposed work-around would be using the Galactic Community to completely ban all forms of slave trading, but that requires the Federations DLC. I don't have it yet, so I can't confirm if it's working or not.
