Dissonance isn't viewed as a shitty game. True, it's typically held as the most meh of the GBA series, but that doesn't mean the game itself sucks.
What killed Diss for me was the graphics at the expense of the gameplay, the rip-off of SotN (from the similar graphical style, similar looking protagonists, the 2 nearly-identical castles bull-crap to the "buddy is possessed and I need to free him" thing), and the more-or-less lack of any real dialogue or plot once you start exploring the second castle. I almost felt like Diss would have been better without the whole (almost) mirror-image castle and just the first castle. It was neat when SotN did this stuff; it was redundant when Diss repeated the same exact stuff that had come before it.
And I don't get what was so bad about the sound. I saw it as an homage to the 8-bit era, the glory years of Castlevania's humble beginnings (and that of the NES itself).