pawsplay
Hero
This is my experience as well. In Living Greyhawk, there were lots of HARD encounters. Encounters specifically built using a combination of feats, classes, monsters, terrain, spells, and so on to defeat any party that wasn't built well.
A particular example always sticks out in my mind: 2 Dread Wraiths and 4 Spectres in an encounter for 12th level characters(EL 15). During this encounter, there was an "evil aura" that made it so no one could see further than 25 feet away from them no matter what light sources they had. The PCs were on a 10ft wide path with forest on both sides that was described as taking 3 squares of movement for each square you moved and doing damage to anyone who tried due to sharp branches and brambles warped and twisted by the evil aura.
Meanwhile, the Dread Wraiths have Spring Attack, a movement of 60 ft flight, were incorporeal, drained 1d8 con per hit, and life sense. This allows them to ignore all the terrain, "see" the PCs even while in the darkness, and move up to the PCs(with reach) and retreat into the dark woods each round. Since they used touch attacks, they hit almost everyone on a 2.
The Spectres are almost as bad. They drain 2 levels on a hit. In one round, it was possible for them to give someone 8 negative levels. They did need 7s to hit most people of this level though, instead of 2s.
This encounter without a cleric is basically impossible.
What, for a 12th level party? Let's imagine the party consists of a wizard, a sorcerer, a fighter, and a monk. As soon as the party figures out what's happening, the wizard casts repulsion. The fighter and rogue ready an action to shift 5 feet as soon as they are attacked, while the sorcerer readies a magic missile to blast anything that makes it past the repulsion. On the next round, the sorcerer readies a wall of force, trapping the next opponent as it tries to spring away. The wizard blasts the opponent with magic missile while the the fighter and monk now whale on the trapped opponent using their ghost touch weapons.
Naturally, if someone happens to know undeath to death, that will make the encounter substantially easier against the spectres.
For a party level +3 encounter, not too bad, really.