For the longest time I thought Dimmu was kinda second rate musician-wise (except for Barker), but after playing some of their earlier stuff, it's quite difficult in places. The tremolo picking is not difficult, but the cross-string tremolo picking is pretty formidable. Emperor is the same way, although more black metal and a bit more progressive.
It's actually easier for me to play "Do Not Speak" from Anaal Nathraakh than "The Blazing Monoliths of Defiance" intro from Dimmu. Now when I say play it, I mean play it perfectly. Anyone can get close, but note for note can be quite challenging sometimes.