The problem is that it is, first, insufficiently foreshadowed, and therefore feels like an asspull. Two optional references, that I'm aware of. And both just mentions of "GF critics," rather than showing an experienced GF user have a spotty memory or something. Or even the heroes noticing other issues beforehand. Secondly, the fact that all of them from the orphanage just happen to all gather in the same strike team is awfully coincidental. I know Cid might have had some hand in it, but Irvine was at a completely different Garden. And thirdly, speaking of Irvine, it's ridiculous that he never mentions it before. Not even by accident when he first sees all the people he grew up with? Why?
It's not unforgiveable, but it's representative of VIII's kind of haphazard storytelling and oddly-behaving characters. Which I think is what turns people off of it.
And...finally, I don't even understand why that reveal even exists. My memory might be wrong here, but speaking in terms of the plot, isn't it only important that Squall, Ellone, and Edea were at the orphanage? What does the story gain by having all of them grow up there?