Its not as great as you say
Advance learning? meh whats so great about that? You cant learn more spells, you get the spells on your list + your advanced learning spell (which has to be a necromancy spell, of which you specialize in anyways). Sure its nice, but its no where near as powerfull as you make it out to be. wizards can pick all the spells you want, your stuck with a very very limited list (Its all necromancy!!) and you can pick an extra spell every 4 levels, which has to be....necromancy! smeh. You loose to much not being a cleric or wizard necro to claim this as awesome as i take you to be claiming it.
The hp? well they arent that great, You loose your con (and fort bonus) right now my level 14 dread necro has a +4 con bonus (with equip). d6+4 gives me an average of 7.5 per level, once i turn lich i get an average of 6.5...... Sure i have a potential for more (but then again, anyone can become a necropolitan, even a mage, and that'd give the mage 1d12hp)
The darkvision? smeh, your level 20? is this really that overbuff? (and again, anyone else could get it being a necropolitan)
The mental bastion upgrade - True, but your now undead. A player could be an undead race and get the same benefits.
+5 natural armor - As a mage with light armor this might seem good, but you dont have any other defensive spells in your repertoir. No mage armor, no shield. Nothing, once your level 20 these level 1 spellslots are just dandy to fill with these spells. Mage armor, a level 1 spell, lasting 20hrs? Again, its not that buff imho
Fear aura - This is true, i dont have my mm but if i recall, its not a free action like it is for a dread necro, a dread necro can activate it a dozen times in one round, and fear effects stack. I'll respond on this better when i dig out my mm
Paralyzing touch - Yeah its nice, but your a mage, and you have to get into touch range, which is not always the best place to be. And considering your spell list you dont have any really powerfull blasting spells to stay at range, meaning warriors (who are usually a cake walk for mages) are gonna be a bit more deadly. (of course till they fail thier save)
+4 turn resistance - Well this says it all right here, your now undead, and in the epic level range, with only a +4 bonus. Downside? YOU ARE TURNABLE!! run into an epic cleric of pelor and kiss your putrid buttox good bye! This is a massive detriment
dr doubling - Yes this is good, but its still bludgeoning, its nothing new, just an improvment.
I think really overall, you do get a lot of class features, and while you are levelling you are pretty balanced to other classes. BUT its not like you get all this stuff immediately for free at 20, you (as its been said) have been gaining it slowly as you level. A lot of it doesnt stack with your class features, so you loose out on what you have, for something else.
And you become undead
Remember too, if your not near your phylacetry and someone finds it *gulp*
Besides, you have to hit 20 levels all in dread necro, no dips, no prestige classes. A bit of payoff for all that dedication is worth it. If you check build pages *expecially on the wizards site* you can find super powerfull wizard builds that all usually have multiple dips. I've even seen a few dread necro builds that look pretty nice, all have dips!!
And... no epic progression! there is no epic progression for your class. Meaning at 20, you take lich, then either your dm makes you a progression for your class, or you pick up a new class.
You trade a lot for what you get.
And personally, imho, an equal level epic cleric gone lich, with the large spell list is definetly at advantage against your 20 dread necro/whatever class you take.
