Volcarona and Dragonite unviable? The usage statistics would disagree with you. Volc and D-nite were #30 and #17 last month, respectively.
I'm just making an example (hell, Dragonite's my favourite pokemon, and the ace of my team). What I'm saying is that a lot of pokemon automatically become less viable (particularly bug, flying, fire etc) just because of Stealth Rock, and that's kind of annoying. At least these days there's a bunch of rapid spinners and defoggers that can generally fit onto your team to alleviate that somewhat.
Stop right there because if you seriously consider "going for the 6-0" part of a healthy strategy then you need to start battling against more skilled players.
Yeah, that was a joke. Though getting 6-0 sweeps is nice. (Rare, though. Haven't had one for ages.) I was around 1700 on the OU ladder before the last reset. I doubt I got there by luck. Anyway, what I'm saying is that I use a lot of offensive setup sweepers. Also, does defog really just remove your opponents' screens? I distinctly remember magic bouncing one and it still removing my screens. Or that could just be my faulty memory, I guess.
Golurk was an interesting case for a Stealth Rock setter, in that being able to spinblocker at the same time. It's main drawback in that area was being susceptible to common offense of spinners, and Defog is a thing, so that niche might as well be considered short-lived.
Golurk was pretty cool! It was a spinblocker, SR setter and offensive tank all in one. I think I remember someone once talking about a theoretical Mega Golurk somewhere and to be honest that could've been pretty fuckin' great. But we can only hope to the fickle beast that is Game Freak.