I haven’t used unleash but it’s pretty clear your characterization is wildly wrong. 30 seconds looking at docs shows it has features for gradual rollouts, A/B testing, experimentaion with analytics, authentication to control available features, and probably a lot more I missed in a quick skim.
If you work on a small startup, not worth it... on a Big corp with a lot of bureaucracy? It's worth it I think.
We have tried self-hosting it, but it's always a pain to allocate people on infra to do it, so I gave up and started paying for it for my teams, and they love it.
They have non-linear scaling complexity, so they work fine with bumping up resources until they don’t and now you’re looking at queueing metrics on Kafka or something equally complicated. They’re horrible to change out because a lot of the APIs aren’t compatible and literally everything uses them.
They’re also awful to maintain. Everything uses them so there’s never a window when it’s convenient to shut down the feature flag system for upgrades or to test backup restores. People are much more accepting of “the vendor will be down for 2 hours on Saturday” than “I’m taking the system down for 2 hours on Saturday”.