Well they do throw a few changes in here and there (like really long 'hypnotic' sections, or parts that go 2,3 or 6 times... )
But the important thing to note is the lack of any change between repetitions of riffs during these 4 times (or whatever), in this area opeth are well below average. I think this is worse than the fact that they repeat riffs the standard 4 times (many bands do this, but most of the good ones change things around and make the repetitions have something unique/interesting about them). But really in opeth's music they are expressing themselves through the everchanging sections/styles, thats what carries the 'story', so for many listeners this repetition wont be an issue.