The other downsides are the deploys are slower, for now at least, and it's still in Beta. Beta means it has SLOs, which are like reliability goals, but it doesn't have SLAs, which means you get refunds for downtime. App Engine Standard does have SLAs:

https://cloud.google.com/appengine/sla

Beta products are also more willing to make backwards incompatible changes with shorter deprecation timelines. But overall it's certainly a lot more, ahem, flexible. Sneaky awesome feature is ability to install things like imagemagick , phantomjs, or ffmpeg, which to shamelessly self-promote I wrote about here:

https://medium.com/google-cloud/scalable-video-transcoding-w...

That's using a custom runtime, which won't be as officially blessed, but you can of course just extend from the official runtimes, since they're on Google Container Registry and open-source:

https://github.com/GoogleCloudPlatform/python-runtime

Just write a Dockerfile that starts with: FROM gcr.io/google_appengine/python

Obvious disclosure again I work for GCP.