Undeath is NOT unlife. Death is un-life. While death is natural state, sad but untained by evil, undeath is something else - a hideous thirdstate - that which should not be.
Creation of Undead
Truly brainless, unthinking undead in my opinion, waste the potential that lies within the undead as an enemy.
When reanimating the dead, one requires a spirit willing to enter the dead husk being used (a live husk implies possession, a different matter entirely). Spirits willing to enshroud themselves in a dead body are not generally good ones, and by far all created undead are irretrievably evil. Demons and their ilk, being the most common evil spirits, are therefore the main driving force in the walking dead, and so they will mirror the demonic in their behaviour.
When creating the undead, the sorcerer performs a ritual that acts as a beacon for an animating spirit, and depending on their nature, convinces, bribes or forces the spirit into the husk. The sorcerer runs the risk, however, of not getting the intended spirit, obtaining instead something weaker then desired, or worse, something more powerful then he can handle.
Some of the more powerful demons can take control of the situation, and even take the corpse and use it to fabricate its own body to fully enter our world. If undead are common, then this is only a remote possibility.
This linkage of demon and other fell spirits to undead helps bind the universe together better in my opinion. One can then create a vast variety of undead, as they all can derive features from the demon used to create them. The turning of undead makes more sense as instead of a brainless corpse, there is a fell spirit being driven back by the holy words of the priest.
In general, due to the countless numbers of restless spirits beyond the veil, it is relatively easy to entice them to enter, but sorcerers will need to balance this against their need to control these creatures. Should control not be a concern, it may be possible to bring back entire legions to shambling unlife, though the sorcerer had best make plans to avoid their attention.
This could even occur spontaneously, and in such cases more powerful beings are likely to be drawn in.