I think the makeup is kind of cheesy and plasticy, but it makes sense he would look different than in Jedi, considering that the scars are fresh in Sith but ~30 years old in Jedi. However, I think the notion that his appearance in Jedi necessitated a catastrophic injury is moronic, especially considering the wound was self-inflicted in Sith. Why couldn’t he just be really fucking old? It is a galaxy far far away. Maybe he was 170 or something. They could have just made McDiarmid look a little older than he actually was in the prequels.