Note "serious work". If you are hiding a banner, an extra button that calls a service method used elsewhere, some text, etc - feature flag away.
Real world problems tend to be, at the very least, "add some new fields to the form" - at which point you are messing with DB schemas OR following bad practices like making fields that should have no business being nullable accept nulls because "who knows if the feature flag is on". This quickly destroys normalization and data consistency.
