I don't know Opeth, but will check them out.
Death may be technical, but so are/were Celtic Frost, Megadeth and a host of other so-called thrash metal bands. It's easy to miss the technicalities of what these great bands do if all you hear is "noise", because you aren't listening properly. It's a bit like looking at one of those 3d posters - you know, the ones where you only see a messy pattern until you look in a certain way, then the picture stands out in a quite spectacular way.
They do fall into a grey area, however, as that style is clearly recognisable as a style in its own right, and I don't really hear them as prog.
If any more progressive Metal bands are added, then Diamond Head (the very first prog metal band) should be added first, IMO.