So recently in our playtest group I tried seeing what it would be like to throw an 'extreme' challenge at them. In this particular case my group is a party of 6th level adventurers with appropriate gear level and three NPCs with them.
They go into this fight after two minor encounters which consumed two heal spells from the cleric and one second level spell from the wizard, as well as a trap that did a bit of damage, so for the most part we're in tip-top shape, and aware that an extreme threat is infront of us.
The creature in question was a Kalavakus Demon (Horned Demon)- I should start by saying it was absolutely masterfully redone from the previous version, the flavor and style of this fight was so much fun.
The problem came from the creature (and others of the level) +to hit versus the PCs armor.
Yes this creature is 4 levels higher and thus 'Extreme' but nearly every time I picked up the dice it was a crit, if it wasn't a crit it was a hit. The creatures AOOs also would often result in a crit - +20 to hit against 22 ac on several of my party members.
While it was still a mostly enjoyable encounter that they barely made it out of, having every attack round not be a question of 'hit or miss' and being of 'hit or crit' was a bit jarring. It was also basically impossible for the monster to fail most save or suck spells to help mitigate this problem.
Have others experienced this in their playtesting? High level creatures seem like they're game enders if not managed properly.