well if you just use appengine without using a vendor lock-in service, i.e. using cloudsql instead of datastore, etc. than you probably won't run into trouble.

well if you just use appengine without using a vendor lock-in service, i.e. using cloudsql instead of datastore, etc. than you probably won't run into trouble. but it looks like appengine still has it's momentum (they actually added java8 support lately)