There are some issues that have not been mentioned.
One is that a group of individually optimized characters can be collectively weaker than non-optimized characters. I remember once in Greg's campaign playtesting Griffin Mountain, we had a large group of about 25 mostly rune levels. My characters were two followers of a Yelmalion rune lord. For reasons that I have forgotten, we split the party. The group on the opposite side of the river was attacked at night by trolls. They were Orlanthi and Humakhti and turned out not to have any light spells {oops}. I can see how that would come about, after all most of the characters only had a certain amount of INT, and it was all used for Bladesharp, Protection, Countermagic, Healing 2, etc. Each player assumed that at least one of the other players would choose spells useful to the entire party, but not the best for an individual character. So they choose the spells that maximized the power of their character in the most commonly encountered situations - and failed miserably against a much weaker force of trolls and trollkin at night.
The second is that very powerful characters who are assigned to something other than a strike mission can be frustrated by enemies who cannot even injure them. A fairly powerful group of PC's, too strong to have random enemies appear and threaten them but escorting a caravan of mules, was very much annoyed by a fairly weak group of bandits. My bandits shot a couple of arrows at unprotected mules, and rode off. The PC's were forced to find a way to carry the goods that were loaded on the dead mule(s). They didn't have the ability to carry the goods themselves, and moved on. The bandits picked up the loot the PC's left behind and a couple of the less skilled bandits took the goods back to the village on their mules. The rest of the bandits got on their horses, followed the PC's, and dropped another couple of mules ...
Another trick can be borrowed from Asian horse nomads. They used expendable ghazis, subject allies, whatever to soften their enemies up. This is a good use for zombies or trollkin. You hit the PC's with these expendables, the PC's put up all their spells, you wait for the spells to go down, and then you launch the real attack.
Finally, players of high level near heroic and heroic and heroic characters can become overconfident. I recall one group, containing all high level characters, that decided to ride straight at an enemy fort and climb the walls. Everyone was much better with melee weapons than missile weapons, and wanted to play a glorious role. As 2/3 of the group ended up pinned beneath dead horses, we ended up parleying with the fort and withdrawing. The characters on the attack easily had more than the 3 to 1 advantage in power necessary to attack, but with most of that power pinned beneath dead horses, we had no hope of winning. Player stupidity can be a very strong equalizing force, but you cannot count on it when planning an encounter
Ray,