I like that you can make your own bubble shapes or use existing ones, and it's easy enough to just click around in an image - no hassle of having to crop out areas first! It would be great if there were more shape types available though (e.g., squares).
Sometimes when making new bubbles they look too similar to each other so as far as visual design goes this is not always ideal. You could potentially have some trouble explaining why someone should choose one over another since all are pretty much identical visually but what makes one better than others? The ability to easily create custom designs without needing Adobe Illustrator experience, which helps me collaborate easier because my colleagues don't need access to such software either.