For us it was the mismatch between AWS and Datadog billing (AWS bills by the second, Datadog bills by the hour, so you should only ever use Datadog for persistent instances, not high churn instances like dynamic background jobs, or else completely rearchitect your application for the benefit of a vendor)
This is incredibly common - I've heard a company end up rearchitecting their instance type choices due to Datadog billing on a per-node basis (with some peak usage billing shenanigans). Their business model unfortunately encourages some very specific architectures which doesn't work for everyone.
