I round the crap out of my upperback with the SSB. I figure it's part of the charm of the implement, as it forces the weight to really fall forward and makes you to have to correct it.
Whenever I pushed up with the handles, it allowed me to arch more, yes, but it also put the weight further behind me.
It's similar to high bar vs low bar squatting I suppose. Consequently, I never do high bar work, and only use the SSB for it.