The reactionless drive attack would have to be a pretty heavy machine to pasturize the planet. Defenses would also be available. It is somewhat analogous to car bombs today.
Even if the efficiency is crap, say travelling at 20 or 30% C, a couple 10's of pounds is going to yield a shitaceous bang.
Something travelling toward the planet at an appreciable fraction of C can also not be "broken up" without the aggregate mass still carrying pretty much the same yield but over a wider area. Particles the size of sand would transit the atmosphere as if it weren't there, like a cosmic ray and impact solid matter like artillery rounds.
You'd better watch out
You'd better beware
'Cause Albert says that E
= MC2
No biggie. Mount the drives and just throw rocks...there's a whole asteroid belt full of 'em.