I would agree that in interviews ADI appeared to show scant regard for Giger's skill at creating his own design. Stating that they could do it better than he himself had. But hopefully they have to follow a director's instuctions, and if Blomkamp wanted something that looked as great as Giger's original then they would do it.
Personally I don't doubt their abilities because I thought the half-destroyed Bishop in Alien3 was amazing, as was their practical stuff for The Thing. But if they are directed badly or left to their own interpretations, especially with something as beautifully designed as Giger's alien, then they are often deserving of the flack they might get, though sharing blame with the director.