I'm not shocked. Slack felt to me like one of the least native, and most "just a browser without all the chrome" of any app I've installed in a really long time. It immediately turned me off.
I don't think this kind of technology is inherently bad, and not everything needs to have a perfect native interface. But I don't think people should be mislead into thinking that this will allow you to make apps that feel just like truly native Mac apps.