Feature Flags are well suited for apps which are being developed across numerous teams in order to extend and basically enable/disable whatever areas of the app you want. It's not really necessary for an individual developer or perhaps even a single team of developers, depending.
Statsig is well suited for running straightforward a/b tests on frontend deployments. We've actually used it to log events on our backend and used the HTTP API to integrate with tools like Hubspot to evaluate email campaigns
For the most part it is pretty easy to use. - There are some quirks with the javascript SDK (getExperiment().getValue?). - The Events vs. Metrics design pattern is complex, and creating new Metrics from Events can be frustrating if you are trying to use event metadata - It's really frustrating not to be able to link Static IDs (before a user signs up) to User IDs, in order to follow users all the way through onboarding, or to log events that occur for signed in users when you are exposing the experiment to users before they've signed up
We still utilize Git as our source control system but Feature Flags allow us to use Git in a less complex and more efficient way for our development needs.
I would say a large ROI considering the reduction in branching and merging we need to perform and also the ability to use common, up to date code across teams. There's a lot of shared code we make use of, particularly with identity and authentication and our entire identity platform which is essentially used by each feature.