Are spot instances risky to use?

Not if managed correctly. This site is powered by 95% spot instances and maintains 99.999% availability. But a single spot instance definitely incurs some risk. If the spot market price goes above your bid, it will be taken away from you, and if that was the only instance powering your application, it is now unavailable while you spin up new instances. However, the more typical cloud app deployment has multiple instances behind a load-balancer. If managed properly by diversifying instances across multiple spot markets, you can isolate the level of impact any single spot market price spike has on your application. For example, if you have your spare CPU capacity target set to 20% and the maximum in any one spot market setting as 20%, then even in the case of a price spike, you have adequate computational power running the application and your users would never notice a difference. And immediately CloudOptimo would start new instances to diversify and return to the target spare capacity level to provide protection from a subsequent price spike. This constant monitoring and diversification logic is what CloudOptimo focuses on for you, allowing you to save money without a lot of work, and focus on your core business objectives.