I think the reason why many fans tend to disfavor the more recent entries OST is not so much the music is bad, cause it really isn't. Rather the shift into voice acting has diminished the music role in the games. In earlier entries, music is used to not just set the mood but help tell the story to the player who can only read the text. The emotion that voice gives is instead utilized by the music instead in earlier entries.
A good example of what I mean comes from FFX, the opening of the game, where you get to see the party at Zanarkand and they play Uematsu's To Zanarkand really gives the player the sense of mood and feeling that words really could not do justice to. Tidus says some profound thing by the end of the scene but the music and visuals have already collided to enrapture the player's heart and soul to the game. After that, the game really doesn't have as many strong music/story ties, because the voice acting is suppose to do all of this and the music is simply a mood enhancer or backdrop of the scene, as opposed to filling the scene with the motive and emotion like older entries.
