One potential weakness is that a declarative rule list controlled by the browser (iOS 9) would be slower to adapt to ever changing approaches use to bypass ad b

One potential weakness is that a declarative rule list controlled by the browser (iOS 9) would be slower to adapt to ever changing approaches use to bypass ad blockers. Whereas the extension model of AdBlock I presume lets it be more clever. But on the flip side, the Safari approach would be much more performant than having to execute some JS to enforce blocking.