One thing I personally realised is that App Engine is absolutely fantastic for my needs, but as soon as my sites are outside of the free quota the costs rise extremely fast almost putting them out of price range before they even got started.

The other thing for me is that I have now spent time and invested hours into learning the App Engine platform but I get almost nothing in return since moving requires me to re-write or re-learn other tools anyway.

Unless I was a 100% committed to App Engine I don't want to launch a new project on top of it.