A couple reasons for which I see developers staying away from necromancy capable characters (and I'm going to assume there's some kind of development or skill-building mechanic involved);
Not only are you tracking the "quality" of the necromancer's abilities (ex a sorcer's fireball at lv 1 does 10+1d4 but at lv 2 does 14+1d4 therefore lv 2 enemies should have 5 more hp to present a challenge) you also have to worry about about the quantity. 2 skeletons doing 1D8 damage each being different from 1 skeleton doing 3+1D4.
Having a small legion of undead at your command, which I would argue is somewhat of the pinnacle goal of a necromancer (or it would be mine at least) would become boring after a while. In the role of a general you could send your army forward to raze and pillage but then it becomes more strategy based and can remove your personal investment from the action. If you add a resource element or some kind of maintenance to the general's role it can become boring. On the flip-side, being a front-line general would be possible, but then you have to worry more about allied AI. Then the questions of "How robust is the AI/What are the possible functions of the AI/What is the player's level of interaction with the AI?" come into play.
Managing the extent or frequency of a necromancer's power can be a pain as well. Do you base their ability to summon critters from the twisted nether on consumable resources, on time in game, or an in-game stat and with each method comes it's own challenges and issues.
Overall, I suspect balance and quality of execution are the more daunting challenges of having a necromancer character, though I would very much like to see a well executed game revolving around one.
*Speaking of necromancy... how the hell did I get to this thread?