It is not applicable to an ice sheet in which preferred orientation of the c-axes is developing.
Ice crystals have a very strong plastic anisotropy.
The effects of this anisotropy are apparent from measurements of surface velocity and bore hole deformations. It is noticed in bore holes that measured strain rates are larger than what Glen's flow law would predict. The problem with these enhancement factors is that they do not usually rely on any physical properties. In the ice cores one usually sees a progressive development of fabric, from initially random at the top to more anisotropic non-random fabric towards the bottom. This has led people to model the development of fabric to explain these observations.