As a Database Engineer working for one of the largest e-company in the market, I can clearly see your point. Definitely AWS RDS is very matured when compared to CloudSQL. I think CloudSQL only provides MySQL and Postgres (still in beta?). So GCE needs to build their Database Arsenal soon.
Next, your client faced issues with replication in GCE, thats not good to hear, but we do face issues in our AWS RDS MySQL and Aurora very frequently. RDS MySQL error logs not generated properly. Aurora has weird memory leaks, connection spikes, starting to behave sporadically when the memory crosses 80% and so on. We are working with AWS to figure out the issue still (credits to the AWS Support for trying to help us). So, to conclude whether you are in AWS or GCE this is the trade-off of "cloud". We need to live that, if you are moving to cloud !!
Cloud Postgres is also hilariously hard limited to 100 simultaneous connections (the default). Doesn't matter how much RAM you give it.
My experience with GCP in the past 4 months has led me to revise my "friends don't let friends use App Engine" motto to "friends don't let friends use Google Cloud", there isn't a single service I touched (except maybe Compute Engine) that didn't have half-baked client libraries, documentation, bugs in the server part, or a complete failure by Google to even have their engs use the competitions tooling before inventing their own shitty clone (DNS)
I was super keen to switch to GCP (for cost saving etc) but this mirrors a lot of my experiences. Deploys to App Engine took 20 minutes, not 2 minutes, and I have absolutely no faith in their firewall settings actually working. I have no idea what the problem is, but it's basically impossible to boot a Rancher master node on Compute Engine. Even with all ports open. In the end I just bailed on the platform as a whole, and I'm moving to a hybrid approach on smaller providers like Packet.net and Digital Ocean.
And I would have been totally fucked over by that Postgres connection limit when we went into production, I'm glad I dodged that bullet! I hadn't bumped into that when playing with dev environments, and I haven't seen that limit mentioned anywhere.
App Engine Flex takes a long time to deploy, and always has. App Engine standard is what deploys quickly, and also scales quicker.
Firewall settings work just fine for our platinum clients with complex network architectures, I don't see why it wouldn't in your case unless something was misconfigured.
Isn't Flex the newer of the platforms? Is there a reason why it's so slow to deploy? I deployed an app via it that deploys in a couple of minutes anywhere else, including build time, but it took an insane amount of time on GAE, and I never managed to find a good reason why.
Normally I'd think I had configured something wrong, except in this case it was insanely simple. A network label that allows all ports both ingress and egress, to any destination/source, definitely applied to the servers, and yet they had constant connection issues with each other.
It probably was something I did, but the combination of those issues, surprise egress bills, very laggy UI, and various other little niggles just made it not worth my time for now. I'm keen to avoid vendor lock-in anyway, so GCP and AWS don't have that many extra features over smaller providers for me.
Next, your client faced issues with replication in GCE, thats not good to hear, but we do face issues in our AWS RDS MySQL and Aurora very frequently. RDS MySQL error logs not generated properly. Aurora has weird memory leaks, connection spikes, starting to behave sporadically when the memory crosses 80% and so on. We are working with AWS to figure out the issue still (credits to the AWS Support for trying to help us). So, to conclude whether you are in AWS or GCE this is the trade-off of "cloud". We need to live that, if you are moving to cloud !!