Look, you don't have to specify Necrons, everything
does it better than TSons. Whatever you're referring to. Thousand Sons are absolutely horrendous, we know.
I'd argue against it, but... yeah... well...
Zooming Flyers and Swooping Monstrous Creatures must also take a Dangerous Terrain test if they move within 18" of an Obelisk, even though they do not normally take dangerous terrain tests.
Eh, it only affects the Heldrake since FMCs have Move Through Cover and therefore automatically ignore Dangerous Terrain, so I don't think it's a huge issue (it's unlikely to do much at the best of times, and on a Heldrake with it's 5++, even less so).
Not sure how that rule would effect it, then.
on the other hand, dropping this thing into the enemy lines like the holy hand grenade sounds fun.