So you obviously don’t build apps for iOS Safari and I can understand as a user why you might be under the misguided impression that it’s a good browser. As our primary target environment it is an uphill battle to get things to work in a way that it’s not on either Firefox or blink based browsers.
It breaks in every major version that’s been released. Simply talk to any developer who has to do a significant amount of work building apps in Safari and they will tell you the same thing.
The webkit team is talented and do great work but this is simply a result of a decade of underfunding and it’s just not possible for them to maintain a modern browser with their current resources.