GCP is 2 years behind in versions and the funky setup around connections is a hassle to deal with. The number of connections is limited and not configurable, and the default way to connecting is to use the Cloud SQL Proxy which is a separate binary/service you have to run to connect.

Public IP access is available but that means going through the public internet, and also means you have to whitelist each individual IP accessing it, which is hard or impossible when trying to use ephemeral or private-ip-only instances.

Cloud SQL itself is fine...it's just hosted SQL. That said, there is a lot of google-bespoke code for interfacing various parts of their cloud with it. Cloud SQL Proxy has a lot of gotchas and hard to debug. For example, the per-instance connection limit between app engine and cloudsql is 5, and it's impossible to know how you're bumping against that. Tuning is a frustrating process, and google support hasn't been too helpful.