I think a big thing most people are ignoring when saying the japanese voices may be inferior is, the comparison is mostly made between the Disney characters...who are quintessentially american cultural creations. Obviously they are not going to sound as good in japanese, that is really just a given, especially when most of the Disney characters are voiced by their original actors, who are usually celebrities or voice actors with tons of experience, whereas Japan probably doesn't "understand" the characters as well and a lot of what made the characters work in the original language will simply be lost. So if you want to compare them it isn't going to stack up favorably 99% of the time, like when a japanese show is dubbed into english. Speaking of which....
Sora, Kairi, and Riku, and all the original characters are all pretty much japanese, so in some cases the voices fit just as well or better than the english VA's. The one I remember that really bothered me was Norio Wakamoto as Xemnas. He's an amazing seiyuu who's voiced a ton of iconic characters, he is pretty much THE villain voice in japan, especially hammy ones, which sounds good on paper, but in my opinion his voice is just way too deep a voice for Xemnas. It sounds wrong.