If you're a startup running something on Elixir (or even Rails), AppEngine's experience is hard to beat.
Not many people do know:
* You can run multiple microservices on AppEngine under one application.
* Each of these can have many versions serving different percentages of traffic.
* There is an on the fly image resizing service, which means, you don't need to run complex imagemagick setups on your machine, instead you can simply call your image with parameters appended Eg. - /my.jpg?size=120x80 and it will be resized on the fly. And it's pretty damn fast, too.
* If you're also using Cloud SQL, you can directly let your app talk to it without whitelisting IPs or even using the proxy, just using sockets.
* You can lock your app under what google calls IAP, a login layer that allows only authorized users to access your app. IF you're building prototype for clients, it's a no brainer and saves you from adding custom auth. (https://cloud.google.com/iap/docs/app-engine-quickstart)
* The development experience is superb, and has gotten better in the last 2 years.
For the record, I tried AWS ElasticBeanstalk recently and it has a lot of bugs, especially with the new interface changes, so I kept coming back to AppEngine. Seriously, if you're doing a startup in 2018, there is no reason to not use AppEngine.