Sacred Haven of Debauchery

well, if you compare it to the classics, everything is "simple"

Well, it depends on which metal you're comparing and with which classic. And also, it depends on what aspect is more complex: in terms of playing technique, composition structure, or understanding. There are quite simple things in the classics (which are brilliant at the same time) that are taught in the first grades of music school, just as there are things in metal that are difficult to play without a lot of experience.
Also, I can say that sludge/stoner is complex because I like these bands, I enjoy immersing myself in their music/lyrics and looking for deeper meaning there; and someone might say that these are just guys who got high on grass and there's no meaning there. It's a matter of taste...
I also love to write a long text and then forget what I started writing about)