GCP is kind of odd in that some hardware is only available in certain zones, and some hardware is technically available in multiple zones but the capacity is not evenly spread. I've had to implement infrastructure code for GCP that interrogated hardware availability by region and zone to place VMs, trading off multi-AZ availability where it made sense.

Yes. The area on the Venn diagram where services, hardware and regulatory compliance intersect is surprisingly small.

I had a client that was contracting with a very large bank and there were only something like four zones globally that we could possibly deploy in to.

Google also is unlike AWS in that their zone abstraction is very virtual, and two zones might be in the same physical building. If you have requirements that your deployment zones be physically separated by some minimal distance then it can make things pretty challenging.