The difficulty is in turning a userland exploit into a useful jailbreak, there will always be parser bugs and WebKit bugs but those are only the first step in the chain. From there you need to escape the sandbox and that's the part which keeps getting more difficult.
The "unhackable" Xbox One has had at least one userland exploit via attacking the web browser, but nothing came of it because they weren't able to break out of Hyper-V.
