Well there's always the elitist crowd that bitches about them "selling out", but a far more legitimate complaint has to do with the way their sound changed in the 2000s. The production on their albums just got extremely noisy and out of balance. The drums especially sound horrible - there's a kind of clickiness to the bass drums. Also their songwriting has gotten progressively worse on each album since Spiritual Black Dimensions to the point that they're practically unlistenable now. I honestly cannot sit through In Sorte Diaboli without wanting to rip my ears off. They sure rocked ass in the '90s though.
Btw, which version of Stormblast did you get - the 1996 original, or the 2005 re-recording?