Ray Alder (Fates Warning) sang backing vocals on PMU in Los Angeles in 1998 and Bruce Dickinson joined them for a couple of Maiden songs: https://mikeportnoy.com/dates/tourography/?show=427
I'm not an Akerfedt fan and I think his vocals are horrible.
Why? He sings perfectly in key and doesn't deviate from the original version.
Eh, it's not always like that. I dig Mikael Akerfeldt, and agree with you, it's a smashing rendition of Repentance, but I have the exact same thing with other vocalists. Neal Morse, is perfectly capable, perfectly powerful, and sometimes really good, but he hits this groove where I always think he sounds really smug. His vocals are my least favourite thing about Flying Colors.
It's a matter of taste. Not a lot of people are going, "I don't like James LaBrie, he's not very talented." Even live, he's one of the better vocalists, and one of the very best at fifty - more often it'll be that there's something about the timbre that doesn't appeal. He's got a distinctive voice, isn't for everyone. Akerfeldt's going to be exactly the same.