With fortnightly builds which are only then released to Engineering, if that is now what is happening, there will be massive instability every 2 weeks, until the final round of bug fixing cycle stops all features being added.
Which can't finish in time because the OS has to be released at an Apple event. Except for 10.0 when the date was known months in advance the old OS cycles were released when ready ( when show stoppers were 0). Of course that did lead to some wrangling about what a show stopper was, but consider if iOS 8 was not released with the HealthKit bugs, and other major issues, and instead we got what was in iOs 8.1, or 10.1 instead of 10.0.
Still some issues but mostly robust. Apple needs to decouple software from hardware releases.