They take half weight for crystal,metal,and obsidian.

Getting a massive TLC update in March 2018. The saddle will act as a mobile smithy, Argentavis will be able to carry with Talons as well as with its beak, reduced carry weight on obsidian and a few other unannounced resources (similar to the mammoth and wood), and will also gain a Rapid HP Regen upon eating (likely from devouring corpses and not specifically food from inventory). Like so people can see!

Thay spawn more near the mountains

The argentavis has a weight reduction of metal, so these would be good for metal trips, they are pretty slow though and picking up creatures can add weight.