In the life sciences space, feature flags were one of the key tools from an engineering perspective to manage complexity and scope of GxP software validation.
We build it right into the software for a number of reasons. One of which is that it's easy -- at its core, just a hashmap. The second is that for GxP validation, it is easier to not have to validate the vendor and only validate the internal implementation.
I am certain that LaunchDarkly has a market where it is the perfect solution, but I have seen it implemented in two startups now (Motion and another $100m C) and in both cases, it felt like an overpriced and overly complicated solution for a simple problem of turning features on/off that the team actually needed.