I saw that tour as well and yeah, Slaughter were indeed god-awful. I find Mark Slaughter's voice nasal and irritating as hell on record, but live it took on a whole new dimension of torture.
Still 'n' all, my "worst" live band would have to be the Cult opening for Metallica in 1989 -- Ian was trashed, stumbling around the stage, mumbling into the mic before he finally gave up and laid down on the drum riser while the rest of the band played around him.
I found out later that by that point in the tour the Cult were so sick of being heckled by the Metallica crowds that they had simply stopped trying...but it pissed me off cuz I was there hoping to see killer sets by BOTH bands. I still love The Cult on record but they've never gotten another nickel outta me for a live show.
I agree with The Cult opening for Metallica. I saw them on that tour too. Bulletboys opened and were pretty good. But Ian was hammered and couldn't sing. I did see them later that year as the headliner, with Bonham opening. They were awesome!!! Great concert to support Sonic Temple. They must have played for over two hours.