What is the difference with a skeleton dragon and an undead dragon? If you were to ask the undead about it then it would be that of the composition of their body and souls. If you ask the dragonkin then they would say that those undead are a mockery of their masters.
An undead dragon that Adrian fought when he travelled back in time is an actual dragon that became an undead that kept its body and soul. The dragonkin will still have an aversion against it because death is supposed to be sacred to dragons as their souls return to the Mother of All Dragons, Tiamat. Once their dragon soul returns to Tiamat, the dragons will once again be reincarnated as an egg or at least that is what the dragonkin are told.
