Yes, there will always be some risks associated with any hosting option. You are relying on the resiliency and engineering of infrastructure that has scaled at an astounding rate.So far, downtime has been extremely limited, and the cloud infrastructure of the major providers has exceeded physical infrastructure (when properly configured for redundancy in multiple regions). However, it’s always possible that the cloud could collapse on itself either due to an attack or even a simple engineering mistake.
Let’s be honest, this is possible with non-cloud infrastructure as well. We’ve had bad routes, upstream providers, and power issues cause more than a few minutes of downtime over the years. The most risk-sensitive approach is a hybrid solution of premise and cloud for highly critical applications. There is a good possibility that whatever circumstances take out a major portion of the Amazon or Google Cloud will also take out physical infrastructure. But simply having physical infrastructure will certainly give you more options in the event of an emergency.