I think you can do some things with chroots[1] that you otherwise couldn't although I could be wrong. My knowledge on Chrome's linux sandboxing (just for comparison) is kind of limited. I'm sure there are other protections such as seccomp which you can heavily restrict in renderer processes that you otherwise couldn't.

[1] With grsecurity they can become much more chroot-jail like. Chrome uses chroot's I believe so it's very beneficial there.