This is a huge success story that I was part of: https://cloud.google.com/customers/gearlaunch/
At our peak, we were processing $500,000 of online orders per day and spending about $5k/month on our entire GPC bill. It was a rounding error compared with our income.
The biggest expenditure was actually on the Postgres database servers that we needed for our analytics, not on AppEngine.
I have used AppEngine in production for about 50+ clients and I am genuinely really curious to know what these limitations are. Maybe it is dependent on the programming language/framework? I run Phoenix/Elixir with Vue.JS + PostgreSQL as standard for most of my clients, it's really a breeze to work with.
In addition, these are the advantages of working with AppEngine:
AppEngine has an enormous number of limitations that you only hit once you scale up and gets very expensive very quickly.