If you have a large relational database and it is growing fast on daily basis, and if you have a strong need in ACID across all your clusters, and most importantly, if you need to maintain high availability in a large region, Google Cloud Spanner can be a very good choice.
Does what it promises well, for instance, as a sidecar for the main enterprise data warehouse. However, I would not recommend using it as the main data warehouse, particularly due to the heavy business logic, as other dedicated tools are more suitable for ensuring scalable operations in terms of change management and multi-developer adjustments.
As with other cloud tools, users must learn a new terminology to navigate the various tools and configurations, and understand Google Cloud's configuration structure to perform even the most basic operations. So the learning curve is quite steep, but after a few months, it gets easier to maintain.
GCP support in general requires a support agreement. For small organizations like us, this is not affordable or reasonable. It would help if Google had a support mechanism for smaller organizations. It was a steep learning curve for us because this was our first entry into the cloud database world. Better documentation also would have helped.
Spanner scales quickly compared to Amazon RDS. Azure Database is about the same as well. MongoDB can scale to horizontal scalability, however, because Mongo doesn't support full ACID, Spanner comes into that aspect. GCP Cloud SQL not as scalable as Spanner. Spanner has more functionality than MongoDB and Amazon RDS & Azure Databases.
Unlike other products, Google Cloud SQL has very flexible features that allow it to be selected for a free trial account so that the product can be analyzed and tested before purchasing it. Integration capabilities with most of the web services tools are easier regarding Google Cloud SQL with its nature and support.