For native apps, anything Jellybean or better is reasonably easy to deal with (and ICS isn't that bad). But for webapps, the system webview was the horrible default Android browser up until KitKat, and didn't get auto-update until Lollipop. Android Browser has a number of outright bugs in its spec compliance (when I was developing for Google Search, we thought of it as the new IE6), and you lose any sort of GPU acceleration or fast JS engine that you might want for animations.