1. At 14th level, you are five caster levels behind in both classes, so if the party Fighter took Leadership, and his cohort got Leadership, he’d actually be bringing more Necromancy to the table than you. As a fighter.
2. You have to take the Death Domain as a Necromancer Cleric, which is a waste of a Domain Slot when you are trying to be good at Necromancy.
3. In the early levels, you postpone your access to Animate Dead by 4 levels.
4. At 8th level a True Necromancer can create, but not control Ghouls. A Cleric at that level can control but not create Ghouls. Guess which is better? At 11th level, the True Necromancer gets the ability to control Ghouls, and the Cleric gets the ability to create them, so there’s no point at which this is advantageous.
5. The only unique ability of the True Necromancer class is unimpressive. Desecrate is a great spell, but it’s also a second level spell.
6. True Necromancers eventually get a bonus to Rebuking – at 17th level they have a +1 bonus to their Rebuking level. But at 7th level they have a 3 level penalty to their Rebuking level. So at low levels when rebuking is good they can’t use it, and at high levels when Rebuking doesn’t matter they don’t care.
7. True Necromancers are always going to have underwhelming Save DCs. Between MAD and the fact that they are often forced to use spells that are 3 spell levels lower than what the single-classed casters can use, they’re going to be out enough Save DC that it shows. A lot.
8. As a True Necromancer you have all the disadvantages of both a Cleric (the gods can take away all your spellcasting at any time), and a Wizard (you have Arcane Spell Failure, preventing you from wearing good armor). Also, your BAB and HPs stink when compared to a Cleric.
9. Control pools from Animate Dead actually don’t accumulate between your two classes. It’ right in the spell, if you cast the spell it considers all undead you control from all castings of Animate Dead, not just your Arcane or just your Divine castings of the spell. Some people say differently, and some even quote CustServ, but when was the last time you won an argument with your DM using the line "some guy on a board said that CustServ told him....."?
10. There is almost no synergy between Cleric and Wizard Necromancy. Any synergy you desperately want to find could be replicated by just taking the Apprentice feat at first level and having some Use Magic Device. Get yourself a couple of Wizard Scrolls or something. It’s a better buy than setting 5 caster levels on fire. Smart cookies can even get the right spell effects off monsters for free, no less.