Google App Engine is very cool. It hides all the complexity of creating a highly available, load-balanced and scalable application. It is a very natural fit for node.js applications. You can quickly and easily achieve “Google Scale”. This is explained in this great video,
There is a synergy between node.js and Google App Engine because node.js is low-level and applications are build by service aggregation.
In contrast, WordPress is a complex stack that does not naturally fit into this model. Google explain the WordPress hosting options and suggest that the best choice is a single instance that you make bigger if you need to.
I will address the challenges of maintaining and scaling production WordPress applications in a separate post but the conclusion will be that it usually makes sense to use a specialist WordPress host because unlike node.js scaling it is messy, unsatisfying and difficult.