There are some features that have yet to be implemented. See the https://forums.spongepowered.org/t/status-update-api-release-edition-not-the-usable-server topic for more details but this explains what is currently missing. I am also fairly certain that like most new software that is released, once plugin developers get to play with it some or even as late as when the first implementation is released, when feedback is received there will be some changes according to that feedback. As of now they plan on doing rolling releases to add the missing features.
Even with these updates I think there will be very little plugin breakage in the future, certainly less than bukkit due to the highly decreased need to use NMS that would break a plugin at every minor version change.