Comment by hn_throwaway_99

The flags (whether they be booleans, strings, numbers, or anything else) are the trivial part. It's the targeting and rollout rules (i.e. who gets to see which flags), and the requirements for extremely fast and consistent evaluation of these rules, that can get surprisingly complicated fast, and folks who have rolled their own usually find that product management or marketing or sales wants to target using more complex rules, and the problem balloons.

I agree that problem is not particularly hard in the grand scheme of things, but it is actually quite big, meaning it requires a lot of features that aren't obvious at first glance.

Edit: Thought of another analogy that may help explain the complexity. At their heart, feature flags are really a permissioning system: only certain users get access to certain pieces of functionality. Anyone who has ever dealt with permission systems know how complex they can be: group membership, including hierarchical groups, roles, ACLs, etc. All of those things are really analogous (actually, a subset really) to the various types of targeting rules that can be used in a feature flags system.