They have never been that impressive as far as i'm concerned.
Simple rythmical progressions on a chromatic scale, unimaginative cadenzas coupled with amateurish singing, not sure why they have become such a cult, the band was never really that skilled.
As for Manowar, well, i'm not sure i can explain that one.
They have a very good singer, and some of their songs are rather tuneful, but for the love of god this band can be so bad at times (the lyrics in particular) they are just histerical.
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Interesting, what bands in the metal-scene have more skill than Opeth?