[TheSpiciestDev]

I've worked with Split.io at a client, they had libraries for our frontend and our backends. Their interface was also neat and would allow you to separate out your feature flags per environment, per user or user group, or through any other custom condition. They'd also provide statistics for A/B testing, but I never got that far with them to try it out.

Theirs was dynamic in that you could change the rules and at a given interval the connected user's app would refresh and react accordingly. It was pretty complex, I suppose, but I'd roll out my own and go to something like them if I had a lot more conditions or targets.