As Abrams using his siesmic impact at max level(therefore being unstoppable), getting hit by knockdown anytime while traveling down toward your target will interrupt the travel and the ult.
I tested with Billy's Bashdown ability (at rank 3 so it's considered heavy melee), and the following items were not applying any bonus weapon damage on hit
Close Quarter
Point blank (the slow effect is applied)
Opening rounds
Melee charge (The cooldown is triggered but no extra damage)
Crushing...
