Originally Posted by J_Byrd
From my personal experience, trap/hex bars tend to create a lot of rounding in the back. I feel its more dangerous of a lift for a beginner than the straight bar.
This is largely going to be related to how the individual sits for a pull. If anything, the lower back should be less likely to round because you begin your pull from a higher starting point. Which means you don't have to squat down as far. With a trapbar deadlift, your torso angle will/should be higher. This is what takes emphasis off the back and places it on the legs.
Not to say that trap bar deadlifts do not work your lower back.