I've only ever done 1 6's pug back in my first or second year of 6's. The general consensus was that I would meet good competition. The result was me top fragging (or at least doing well) with some other person while some 12-15 year old and some 20 year old or something bitched at each other. Never did them again after that. (Not stating that this is the general pug, just saying that it was a reason to add unto the fact that I was busy with HL.)
As for 9v9, pugs pulled a full 180. Back when I first pug'd, before Mix was a thing, the quality in terms of skill was pretty low. As a fresh Silver player, I would top frag, so they were pretty lackluster, but most people tried at least. Nowadays it's almost the complete opposite. No one works together, the majority of the time I'm sitting on the flank by myself while the Scout and Soldier try for dumb plays to try and look good. Pretty much no one takes them seriously. Don't get me started on Pay Load pugs, either, I stopped doing those a long time ago. There is no reason to PUG as an Engineer or Medic unless you are trying to make a name for yourself. They are not fun or productive, simply a waste of time.
HL Mix (and apparently 6's mix) seem to be counterproductive. I've never done 6's mix because of the stories I hear and the experience I've had over in HL mix. There is no effort in HL Mix. None. No one is looking to get better (the whole point of it existing is to get better so you can do real pugs), it's just a bunch of people looking to goof off with some Gold/Plat/Invite players looking to off class because instead of sticking to normal pugs, they'd rather off class. In turn, mix not only encourages bad behavior, but kills the normal pugs.
In-house pugs (HL) have not been successful, either, people still do dumb shit.
I tried TF2Center as well, but those are even worse. Half the people don't enter mumble, no one talks, people bitching at each other in Team chat etc.
People need to tryhard without being a shithead about it. If people aren't trying, then there is no reason for PUG to exist, but at the same time, being tryhard to the point where you are bitching to someone an entire PUG is counter-productive. I don't know what's so hard about grasping this concept with this community. If you don't want to get better or try, don't add up. I'd personally rather have PUGs dead then waste my time.