Proto.io is an excellent product for small to medium teams that wants to have a prototyping or wireframing tool that is easy to use, can be used in collaboration with others and will deliver superior results with less efforts than similar options in the market. I think it is not so suited for teams that don't have more complex prototyping needs. In this case, other tools that are even simpler could be a better fit.
Proto.io is web-based for one, which is a plus - Adobe XD is a desktop application. Adobe XD has some great features, especially for a free product, but from what I experienced Proto.io had way more tools and customization for creating interactive prototypes. Adobe XD is good for building prototypes that go from screen to screen, but Proto.io allows you to get much more detailed with that (every element on the screen can have an interaction).