At the danger of oversimplifying sings severely. While WebKit is open, the rest of Safari is closed source. In this day and age it should be the most common heuristic that you should always prefer more open software over less open (aka closed) software if you have a choice and security matters to you.
Sometimes it's not practical (otherwise the advice could simply be "just don't use OSX") but for browsers you do have a pretty decent choice.
I haven't exactly measured is but Apple patches for Safari also seem fairly slow compared to Mozilla or Chrome so I agree with that (same argument posted on the github)
Not in any effective way. I'm pretty sure you can take Chromium, replace the logo and branding, compile in mp4 support and drop in the 2 plugins (ppapi flash, and ppapi pdf viewer) and you'll end up with Chrome. No difference
Sometimes it's not practical (otherwise the advice could simply be "just don't use OSX") but for browsers you do have a pretty decent choice.
I haven't exactly measured is but Apple patches for Safari also seem fairly slow compared to Mozilla or Chrome so I agree with that (same argument posted on the github)