We have a educational system (federated microservices that are running mostly on AWS EC2, RDS MySQL and a message queue) that is already distributed across regions, primarily for latency hiding, though it has been useful for failovers. We're now considering adding HA within each region, by adding active-passive failovers in using multiple AZs in each region. However, we received an attractive offer from another large IaaS provider to host the failover on their infrastructure at a very competitive price point.
What could be the downside of having a deployment that uses multiple IaaS service providers? There isn't much literature out there from real-world deployments.