It would have a few side effects beyond the durations: Damage Scaling. Think most Necro spells are capped quite low - but a few I can think of could be nasty - Horrid Wilting would do 25 Dice for the above guy... although that is not earth shatteringly different - about the same as the meteor swarm a normal 18th level mage could use.
SR would not be a good defense against the True Necromancer/Theurge either. Taken to that 28 CL extreme I put there: SR 39 would offer 100% protection against an 18th level casters but only 50/50 against the True Necro.... He'd pretty much ignore SRs apt to his level. Could cause a few upsets?
Although on the flip side, it would also fit quite nicely with the True Necro concept - weaker as both a cleric or a wizard, but truly epic as a necromancer. It's also more of a 'DM class', which some might say makes it less 'problematic'.
If they spot this one, I think it'd probably get changed. Personally, I'd like to see the Theurge spell progression used for the True Necomancer. Then use character level as the caster level for necromancy spells. Although that could possibly have some funny side effects as well!