A Predator's bio-mask comes equipped with X-Ray vision and therefore by that logic, they would be able to detect the adamantium bones as well as the weak spots in the skeleton, such as the joints and tendons. Because of this, the tendons and disc in the neck can be cut though one would need either good precision to do it, or surgical precision. They are left uncovered and still soft otherwise Wolverine would not be able to move his head and neck if his joints were completely covered. What I mean by that is that the disc and tendons between the grated neck bones are at all protected. Decapitation on Wolverine is in fact possible as the vertebrae in his neck bones are not necessarily protected. The Adamantium skeleton is what makes Wolverine extremely durable, but there are chinks in the armor as many would say.