At the beginning of the 2015 I worked on a python application in Google App Engine. After all, because of some limitations and vendor locks, the app was moved to docker environment. Here is the list of these limitations. I did not check since then, maybe something changed, there is App Engine Flexible Environment, but it’s still in beta.
It’s not possible to send logs to the external service, the interface in the google console is slow and ugly. The good thing is that it groups logs by request.
MySQL is limited to version choosed by Google, at that time it was 5.6 and no information about when it will be updated to 5.7.
MongoDB does not exist, no possible to use external instance through direct connection, only through REST.
Instead MongoDB there is Google Cloud Datastore, it’s a vendor lock and i did not find a way to work with a document without schema.
For cache there is a memcached from their lib
google.appengine.api.memcache, for flexibility better to use a werkzeug cache wrapper.
Tasks queue is also a vendor lock, with some efforts it can be replaced with celery.