Beastmaster units are very deadly for their cost. Fragile yes, but it comes with the territory. Been on the receiving end of one and it took me awhile to kill them because my opponent was a champ at rolling 4+s...
Another friend of mine is running a min-strength Grotesque unit, with Flamer, and Flesh Gauntlet Aberration and attached Haemonculus, stuck in a Raider. 3 unique models, and useable as a suicidal melee unit to punk stuff like Nob Bikers, static vehicles or Tyranid Monstrous Creatures. Meep.>>12576966
Pretty much everything in the book has its place. Barring Derpcrapitator.
That said, one little problem with your little Wrack setup:
-A Venom only carries 5 models.
-A Wrack unit needs 5 models to take a Liquifier.
-To have 5+a Haemonculus, would require a Raider.
Even if this weren't the case, placing an anti-infantry unit in an anti-infantry transport is bad, for it simplifies target priority your opponents ("Metal Boxes ignore this unit..." or "Infantry horde targets *this* unit above the rest")