Well I can see a major mark against it: it relies on an accessory that costs more than most games do and has low popularity. Also, I think you didn't read any press release on Wii Music, erm, at all. Multiplayer is a prominent feature, and it's quite possible to share songs between Wiis. And the lack of any kind of inherent "avatar" system in the PS3 would make the whole experience feel disconnected; perhaps that could be remedied with Home's avatars, but only if they integrate them in such a fashion that any software can access them.
Even with all that, the game still wouldn't do well. Were it designed with the Wii Music philosophy of making music instead of playing notes, it would be panned by the majority of the PS3's existing audience and be overshadowed by Wii Music simply because Wii Music exists on the Wii already and is a much cheaper alternative. Were it a "play notes" system ala Guitar Hero/Rock Band but with more realism, the hardcore would likely eat it up but it would be in direct competition with Guitar Hero and Rock Band and thus lose out on many sales to them.
Sky Render - Sanity is for the weak.
