Solid article. I've worked with complex feature flags / gating systems for years at multiple large internet companies and have largely come to the same conclusions.

That feature flags let one version of the software run in a combinatorial number of modes is both their superpower and kryptonite. Use them wisely and clean them up as soon as possible.

One problem I've seen happen over and over again is when people pile feature flags on top of feature flags. With the systems I've used, all the flags can be independently enabled by the gating service, but that doesn't mean that every combination of flags is a valid state for the program to run in. If your system will break if flag A is active while flag B is not, then it's worth the effort to write an abstraction that checks both flags and fails to a valid state.