There are several problems with implosion missiles. One, the only place you can take them is on the Voidraven which most people don't like. Two, they are only small blast so they will scatter off your target easily and won't hit that many in the first place. The things that you should be firing these at generally come in small blobs, maybe five, so if you scatter its a waste. Third, you can still take cover, which in this edition is mostly 4+, so half will survive. Fourth, how many multiple wound guys that aren't HQs do you really see? That would be these guys main strength but most of those things you could instant death with lances. Fifth, they cost 30 points each! A voidraven kitted with these will cost you 265 points, more than a Land Raider with armour 11. People will just laugh at that... Sixth, a Ravager with 3 disintegrators would do better for less and honestly a bunch if venoms will be even more useful!
So now you've seen some of the downsides, can you see why people don't take them. Really if you like that sort of stuff a hexrifle would do you much better as its the same thing without the blast but you can't take any saves. Do yourself a favour and don't take implosion missiles...
Darkness falls across the land, the midnight hour is close at hand, creatures crawl in search of blood, to terrorize y'alls neighbourhood.
And whosoever shall be found, without the soul for getting down, must stand and face the hounds of hell, and rot inside a corpse's shell.