Ermz
¯\(°_o)/¯
Aside from the vocal style, I see hardly anything else that would link Opeth to Death Metal. The riffs are too intricate and too melodic in nature to be considered Death Metal... something one would pick up learning their songs on an instrument... the riffs themselves dont sound 'heavy' alone.. they usually need the full complement of 2nd guitar harmony and rhythm/bass to make them sound 'full'. I would consider Opeth Progressive Death Metal, but only 'death' because of the vocals and the tendency for the music to climax at times to the point that you feel you may be listening to blast beats (which is obviously a clever deception).