I think you give them far too much credit. If you watch the making-of docs most of the design ideas are their own. The director might ask for a certain look or colour scheme and to tweak small individual features, but the bulk of the alien designs are ADI's own and they're awful. Gillis has even said they purposefully make changes to put their own spin on the creature. Alien 3 looked okay. But Resurrection was horrible, and that was not using re-used moulds. The necks of the creatures in that one are just one big congealed mess which were eventually fixed in AVP Requiem and still looked poor.
And look what they did to the Predator. Granted Anderson wanted them bulkier and with more armour, but ADI did the faces and of course decided that Scar needed to be more heroic and look how that turned out. I won't even go into the rubber dreadlocks and plastic wrist blades.
I've seen their work on other projects and it's fine, but they cannot do justice to the Alien and they have had enough attempts at trying now. People defend them seemingly because there's nothing else to compare them to when it comes to Alien, minus Giger. It's time someone else was given a chance.