Mobile Chrome + Mobile Safari would be a great way to cover enough bases to make a good responsive web app reach a huge audience. The core problem is that Mobile Safari is really quite frustrating in a lot of ways.
As a simple example, I had a bug with some of the dialogs and menu popups I was using not rendering in Mobile Safari. It turns out that if you have a div that is a child (in the DOM) of a div that has overflow: hidden, that child will not be rendered outside of the clipping box of the parent, even if the child div is position: absolute and at a higher z-index than the parent. This works differently in chrome.
There are plenty of workarounds available, but the basic strategy of creating a position: relative context and having a position: absolute floating menu / dialog that's rendered near the button that creates it won't work if your menu bar has overflow: hidden on it. But then you have to make sure you've got your menubar set up well so that it doesn't become super-tall in narrow screens, because you specified overflow: visible. Or you have to put your floating divs elsewhere in the DOM tree and specify their position as fixed and manually calculate where to put them using javascript.
It's things like this that frustrate me the most in working with safari - I'm constantly wrestling with a rendering stack that doesn't seem to do what I want (it was only in recent versions that I could stop setting flex-basis: 0.01px instead of flex-basis: auto (on safari) like I do everywhere else on divs that had only text children that I wanted to make expand but also become multiline text instead of pushing everything way out to the right. And don't get me started on safari's support for indexeddb, let alone the other neat features that chrome is supporting to make mobile just plain better.
I'm at the point now where I'd literally be willing to tell my users: "install chrome for iOS" if it were actually chrome, instead of supporting safari. But instead, I deal with the sort of resonance back-and-forthing when I fix a safari layout bug that introduces oddities in chrome's renders.