App Engine is really awesome! There's a few caveats I would add if you're using it for Rails or Elixir which requires the Flex environment.
* No integration with Standard APIs
* Slow / build deploy times
* Minimum pricing of $40 per machine / month
Overall, I feel like the direction App Engine is headed makes it purely better than other PaaS services but if you're running a low traffic website that isn't standard environment (Java 7/8, Python 2.7, Node 8, PHP 5.5, Go 1.6/7/8/9), Heroku is probably still more economical and better developer ux.
I hope they add Ruby and Elixir to the standard environment. I don't think any other PaaS would compare.