Sorry. I don't buy that just because the feat is rarely taken in one campaign indicates balance.
There's lots of differences between the campaigns.
I'm saying that compared to feats found elsewhere, Spellcasting Prodigy has an edge on them. It is more powerful than a spell like Extra Spell. It's similar to taking a Kinda OK Spell Focus feat that gives you a +1 to all DC's, AND having an Extra Spell feat.
So it's good.
The FRCS is, as I've said, kinda riddled with these feats.
They're not going to unbalance a game. They're not going to destroy all your hard work in a campaign, or probably even be significantly likely to be great. But they're good in comparison to other feats. This is what makes them rather unbalanced.
Really, it wasn't this feat that made me think the FRCS was balanced a bit wonky. It was the feat that gives you a +2 bonus to Ride AND a free martial weapon proficiency.
That thing is two feats for the price of one...ergo, better.
Is it really going to make that big of a difference? No. If you're a first level and you're going for a horseback bowman, you'll probably be mostly a Fighter anyway, so the maritial weapon prof. is just redundant (skill focus would be better). If you're trying to, say, max out your skill ranks for other things and you've taken LV1 as a Rogue, it's better than taking simply Maritial Weapon Proficiency, if you want to be a bow-wiedling horseman.
Really, it boils down to the feat being better than normal, and that not really ruining the game in the course. If you have to be told it's better than normal, I'm not sure you've compared it to others. If you're convinced that, even if it is better than normal, Spellcasting Prodigy really doesn't unbalance that much, you'd be right. 100% right. +1 to DC's and an extra spell aren't going to suddenly make you kick a whole lot more bootay. But it is better than +2 to a school OR an Extra Spell.
The only thing I fault the FRCS for in this respect is that it was the first campaign setting, and it broke the rules simply because it was cool and it could. It set a bad precedent.