The composition styles are radically different, with Japan/international's soundtrack having a melodic emphasis and taking inspiration from various dance, j-pop and electronic music genres, whereas the U.S. soundtrack has more of an emphasis on atmosphere with a rock influence.
The lack of a musical thru-line between the past/present/future tracks is in my mind the biggest weakness of the American OST, and since there are no American past bgms, they stick out like a sore thumb. I would also argue that Sonic music is necessarily melodic, and since in the U.S. soundtrack the melody is a lot of the times dialed back or not even there, it almost doesn't sound like Sonic music a lot of the time.