Unholy scions are the result of humanoids (or rarely, animals) being possessed by evil forces while in the womb, causing them to develop a sadistic disposition as well as supernatural powers. Because that's not quite creepy enough, the powers manifest before the child is born, and its mother is unable to do anything about it despite being aware of it all. Oh, and the infant soul is irretrievably consumed and lost in this process.
That said, I'm going to assume any unholy scions played by people will be at least young adults (or at least, using the stats of one), simply because I'm not aware of any child or fetus statblocks one could apply the template to. It makes it a lot easier to rate, that's for sure.
First of the template's features are a type change to Outsider with the Native and Evil subtypes. It's all of the FoP-qualifying goodness without the banishment vulnerability, what's not to love?
Scions also get to add their charisma modifier to AC as a deflection bonus (obviously great), are immune to poison and mind-affecting effects, get SR 10+HD, have some energy resistances, and receive Fast Healing 4. There's also DR 5 (that increases to DR 10 at 12 HD) but it's overcome by magic or good, making it not that useful.
Less great are the natural weapons (1d4 claws, notable only for being indistinguishable from normal fingers). They don't even get two claws, and apparently can't use it with a one-handed weapon either.
Ability-wise, the scion's mental scores improve considerably (especially intelligence), as does its dexterity.
Scions get several special attacks and qualities worth noting. Familial charm is unrateable, because depending on the base creature you either get a worthless commoner or a dire tiger for a companion. Unholy Strike is interesting, and pretty cool against good opponents, but not a major factor (remember, Evil characters are still quite likely to fight more Neutral and Evil opponents than Good ones).
Finally, there's the Scion's numerous SLAs, which function like a half-fey's or half-fiends: the more HD it has, the more of them it can use. Overall quality of the SLAs is pretty good (Baleful Polymorph, Gate, Major Image, True Seeing, Protection From Good).
Comparing this template to the half-fiend, it gives a slightly better type, doesn't grant wings, gives a better AC boost, has worse stat adjustments, slightly worse DR, better immunities, and better SLAs. I think +3* is completely okay to assign here, but do discuss.