Thorn Carapace is, IMO, not UP at all (actually I think it is slightly OP). It is almost like Fire Shield/Buckler with the exception that it works 75% (instead of 100%) of time and it poisons the target. It may be destroyed in a few turns due to PC, but it just needs to live long enough to poison an enemy creature and the poison will kill the creature through time. Also, its CC effect is cumulative with other poisoning (even new poisoning from the very same Thorn/Spine shield), making even the toughest creature to die relatively quickly by severe poisoning. The
element is actually completely wiped by this shield if it doesn't destroy it in time. I have killed countless Ruby Dragons or Seraphim with it and I really recommend it to many decks as an effective CC resource, able to affect even immortal creatures!
By the way, every single shield (with a few exceptions) can be destroyed via PC, that doesn't make Thorn/Spine Carpace useless.