There are a few open-source feature flag libraries out there, but what makes our service better (IMO) is mostly the cross-platform consistency (so a user will get the same value of a flag across different services, mobile, etc) and the ease with which non-technical people (eg product managers, etc) can adjust the targeting.

Of course, you should evaluate the options for your situation. For many use cases, an open source library will solve the problem just fine. For larger projects/teams, something like LaunchDarkly will be a better fit.

The biggest challenge in implementing this is handling scale. To be able to do feature flagging for the largest web products you need to be able to handle the scale of those products.

Any of us could build this ourselves in a few minutes for a local app, but to scale up to millions of users is very tough.

I'm working on that. Currently we are using LD but it's had some issues, enough that their CEO sent email and refunded some of our money.

As far as Scale is concerned we routinely see request timeout to LD and then suddenly a user loses a FF they previously had. Our cached value for the FF expires and then LD fails and then my customer is mad