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Non-intelligent undead get more HD for their CR than intelligent undead, making them more resistant to turning than the free-willed undead.
Maybe that's by design, but it seems counter-intuitive to me. I think that's an unintended consequence, myself.
The Gray Render zombie in the SRD has 20HD... and it's CR6. Such creatures also typically have a Will save more than double their CR. The Gray Render has Will +12.
Compared to the CR7 Spectre: 7 HD, 45 hp, Will +7, +2 turn resistance, CR7.
I would prefer to see the turning mechanic unhooked from HD/hp directly, and perhaps make more account of intelligent/non-intelligent undead status. If Turn Undead is dramatically more effective against non-intelligent undead, this helps mitigate the HD/CR discrepancy.
I would at least double the damage (or disallow saves for) non-intelligent undead.