I think you've nailed it re: chaincannon havocs. Still, if it's against an army with an interceptor strat, they're gonna get on the turn they drop and all those points will be wasted. I know loyalist astartes and eldar can intercept, I'm sure others can too, but it is certainly a hard counter.
From the core rules FAQ:
Q: If the unit arriving as reinforcements has another unit
embarked inside it which must disembark after it has been
set up (such as units embarked within a Drop Pod, or
a Tyrannocyte), can the firing unit shoot at the unit as
A: No – though the unit can shoot at the Drop Pod/
Tyrannocyte before the units inside disembark.