But then there's the prog rock too.
Metal has its share of time-signature fetishists and intellectual posing, but the prog in play here from Opeth is the gentle existentialism of Dark Side-era Pink Floyd. The pianos, acoustic guitars and ballad-like interludes sandwiched between the death metal crunches not only give the loud-quiet-loud song structures real dynamism, but also make for a more adventurous but less brutal listen.
Lynchpin Mikael Akerfeldt has a good Greg Lake/Roger Waters style 'clean' voice in between the Cookie Monster moments and sings about mental frailty and personal crisis rather than horror movie scenarios. The album artwork is appropriately gothic rather than the usual bloodstained DM fantasies.
In short - nothing about Opeth scares the horses or implies that by listening to death metal you're suddenly regressing to being a 16-year old bedroom-dweller with a large collection of Space Marine figures.**
Progressive rock's more doubtful gift to Opeth is sheer length. Deliverance, which I've been listening too most recently, has six tracks, five of which clock in at 10 minutes plus, which in theory means a lot of tunes per track. This works in spades for the title track, which takes you up a switchback of folk and metal before culminating in twitchy death metal funk with - yes - a tricky time signature. It's utterly brilliant, as is Wreath, the number which kicks off the album.
The rest of the prog/death mash-up on Deliverance doesn't work quite as well - with the benefit of hindsight it feels like a rehearsal for the superior (if a little softer) Watershed. The length of the tracks start to work against Opeth, with the two different strands like oil and water, refusing to come together.
Opeth's other weakness is the flipside of their accessibility - I wonder that by giving up on the bug-eyed, gross-out quality of death metal to deal in Floydian platitudes some of the intensity is (rightly or wrongly) lost thereby. They're an easy band to like and respect but a hard band to love.
* there's Damnation too, and that's all prog rock balladry, presaging Opeth's later career trajectory.
** I had Space Marines once, admittedly. But I gave them away to the Space Marine Conservation Society.