All the points he makes are valid - and stem from the fact that GAE is designed to be 'infinitely scalable'. Because of this, it forces you into a number of design patterns that facilitate distributed, scalable software.

This can be a good thing, if you know scalability is going to be a killer feature in the near future. It can also a real pain in the ass if it's more important to simply get something off the ground quickly and see if it has market traction, and you don't want abandon the convenient but difficult to scale practices like long running processes and JOINs. In my experience, most startups fall squarely into this latter camp. Scalability is a nice problem to have for most of us.

AppEngine for Business now has a hosted SQL mode, which presumably uses a less scalable but ACID compliant alternative to the standard GAE data store (disclaimer - I haven't used it). Since he's already throwing down some serious coin on his app on GAE it might be worth investigating that before abandoning the platform completely.