I just deadlifted five plates for the first time in my life. With a trap bar. I love it!
Why? Superior leverage. Also, no need for a mixed grip. Having the neutral grip on both sides of the trap are on your left, and right hand is more optimal.
Also, being able to put your body in the center of the trap bar makes more sense, instead of having the barbell in front of your shins.
Therefore, the greatest upside is that you could lift more. Better force transfer.
Achieving the 5 plate deadlift — now I can die happy!! Perhaps one of the most personally meaningful achievements of my life.