I think there is some functionality such as google.appengine.ext.db (not ndb) which is not directly available, though? And task queues seem to be completely different. But I accept that for many applications, there are close alternatives.
Edit: And as a point of reference, our application has hit basically every single GAE limitation/quota over the years, including the 10,000 file limit (mostly source code files). So migration is a serious concern.
