I haven't noticed anything strange about Burton's keys on The Sac live, but I'm sure it's a hard one for Ville to sing. I remember some 2004 performances of the song that aren't so good.
The guys are VERY tight players, they never seemed to have much problems with the songs. Of course they make mistakes, but that's a different thing. Back then when Gas wasn't using click tracks, some tempo changes would happen during the songs sometimes.
One of the most challenging songs is probably BABL. I think it took a while for Kosmo to get the kick hits right. Songs with faster tempos are usually harder to play, for example Soul On Fire, Under The Rose and the faster songs on Screamworks.