pugs...
For new players that want to start doing speed clears I'd say get your monk or nec there and experience it first. It is easier to get into it that way and harder to fcuk up a run if you're the caster. Just be honest and tell your Vale Sin beforehand

most sins will help you and tell you where to stay and when to cast. And dont feel pressured into trying to complete your area in a record time. Once you get the hang of it you can go for speed but before that please please just focus on the task and do it right.
In 90% of the pug runs I've been in the Mountains sin died. Of all the areas it is the most difficult one so I dont really complain too much when it happens.
The other areas are really easy once you've done it a few times.
I've done all areas and from what I've seen the level of difficulty looks something like this (easiest to hardest):
Wastes - such an easy run it's laughable. just MAKE SURE the area around the Ice King is clear please. That's it. You cannot fail. But some still do...
Plains - this is chaos plains farming, pure and simple. Nothing hard here except for the occasional bad Charged spawn on the way there. Plains seldom die though and when they do it's also not really such a big deal.
Pits - again, a farming level difficulty run... but amazing how often they fail. It seems they get too confident with terrorwebs and they dont always kill the group behind the monument. If they die before taking the quest it's ok. Farm time for someone else

Chamber - Boring. if they dont die before taking care of the first three Keepers and they dont get the reaper killed the run should be ok. It's not the easiest job but because you can practice this one on your own (with a small investment in celerities) chamber sins are mostly ok.
Pools - blacknesses tend to make it difficult sometimes. Time your IaU well so that you dont drop SF. Try not to aggro too many of them, even if it makes your run a little longer. With practice this one becomes easier but, again, you can never be sure how experienced a player is...
Vale Sin - They fail frequently. Then leave...

Anyway, I dont get why this is so difficult? The pull in behind the rock is tricky to perfect and if you screw up here and pop the reaper you will most likely be kicked out. But the most difficult part here is just not to get the spirits killed! Oh, and Heal yourself dammit. And take res scrolls for your caster.
Mountains - I hate it. Yes, this one is tough and depends a lot on the spawns you get. Charged on the way there shouldn't be a huge problem, again, you just time IaU and SF and kill them. Finish Him on behemoths and that should be it... but mostly it's not that easy. Usually when mnts die nobody complains.
None of these areas are for beginners and the simple truth is there is just no way of knowing how experienced a person is when forming a group. Checking titles is not the solution because even with a high rank the sin could be completely new to speed clears. You just never know.