I'd say this is a job for feature flags. That way you always have exactly one live version of the code, but still retain the ability to hide WIP from users until it's ready.
If you're instead doing this with feature branches or something like that, then by definition you don't have CI. You have NI: Never Integration.
Because, to an approximation, there's never any point in time where all of the code you're working on is integrated together so that everyone has a chance to see how what they're doing interacts with what everyone else is doing. And yes, it is possible for a branch to successfully auto-merge and produce something that compiles and passes all automated tests, and still introduce a horrible regression defect because of an unanticipated interaction between two different changes on two different feature branches. I don't see it happen often, but when it does it usually creates such a big production SNAFU that even once every 5 years is still way too often for my taste.
