HM should not disappear if Doom gets hit. It would turn the assist to a combo extender almost exclusively.
Doom stays on the screen longer then probably any assist. If you watch the average match with Doom Missiles being used, that assist gets interrupted alot, if the missiles disappeared you would barely see em in any match.
the best "nerf" i can think of for HM is either dial back the tracking a bit or cut the number of missiles down(maybe down to four)