This is something that came up in a practise game I was in last night.

Earthshatter travels under the payload, and the hitbox goes up through the payload. If you're on the cart holding shield, the enemy rein can just use earthshatter under payload, and it will catch you and any squishies you are shielding.

This goes for some other abilities as well. I've had some hilarious Hanzo kills when the rein was sitting on the payload. Scattershot under the cart at low angle to catch the healer taking cover behind it, and the rein shield can't stop it.

I'm not going to tell you where you should be standing, but if you're on the payload you're leaving a huge opening under the payload that enemies can take advantage of.