It’s only “behind” relative to Chrome, the dominant browser which more or less sets the standards these days. Calling Safari “behind” is judging browsers by how similar to Chrome they are.
Also Firefox and even Edge when the IE version was being developed. They've also refused to implement features which would bring web apps in line with the the capabilities of native iOS apps (things like push notifications, offline support with data that doesn't get cleared every 7 days).
Safari is the IE of browsers with respect to being buggy and behind on features with a slower release schedule.
IE was both and more extreme on both issues.