Muffs are actually a very versatile pedal. fuzzes are just difficult for some, some hate em, some can't be without em, but love em or hate em, they're hard to get the hang of which is why a lot don't bother. I actually consider the muff a distortion pedal.
They're not suddenly experiencing a revival though, they've just come on your radar and your noticing them more ;)
I have a bunch of muff, some of which i want to get shot of, because there's only so much muff you need, or can cope with. I made the mistake of putting the muff on a pedestal, and muff ruled my life for a while. But finally i withdrew from the muff, exhausted.
Juans right, muffs are variable. a lot of the different kinds haven't been intentionally revisions, the circuit just constantly evolved into distinct types. everything else was in between. the only one separate/distinct in sound to the hereditory lineage is the IC/opamp muff. and it still sounds muffy