Does a dental night guard worn by an adult always need to be made in very hard plastic around teeth or can a more pliable softer one sometimes be ok?

Kind of: I like a night guard that has a hard top(surface that faces the opposite teeth) and a softer material that rest on teeth that support the nightguard. This seems the best of both worlds. the softer material is easier to fit into place, and provides some wiggle room for the teeth. The harder surface can be adjusted for specific occlusal needs and doesn't wear out so fast.

Absolute question: No, a bruxism appliance does not have to always have to be hard, or soft. Each situation is slightly different, as are the treatment objectives of the dentists who are taking care of you. As your dentist what is best for you

Night guard: Full coverage hard acrylic appliance have the most scientific support. Offer a distinct advantage over other appliances because they control and maintain tooth position and for the prevention of bruxism and TMJ syndrome through reduction of trigeminally innervated muscular activity. See orofacial pain specialist for evaluation and treatment.