There would be some significant limitations, the way the cameras currently work.
No button for your guests to push when they arrive. Motion/recording alerts are sent to your phone only after the camera finishes recording a video; depending on how long you have set videos to record, someone could be standing outside for up to a minute before you even realize they are there. And there have been issues with audio and poor quality of live streamed video, which are hopefully temporary as they try to push out fixes.
They could potentially release a button that pairs with the camera in the future, and instant notifications are not beyond their capabilities (that feature did actually work during a beta test last year). But at this time, I wouldn’t really consider it much of a doorbell replacement/alternative.