In 3.5, at level 16 you are ambushed, at night from 400' range 32 level 6 wizards casting fireball. That is about 32*6d6 = 32*6*3.5 = 672 average damage, or 336 if you save for half. The encounter is EL16 so an appropriate challenge. The whole party will probably die in the surprise-round...
...
This is typical 3.x encounters that are totally broken because the system is so swingy.
My experience certainly varies, generally because I followed that bit in the DMG that expressly recommended that CR's stay within a few points of the EL. 4e has a similar blurb, and I think you'll find that a ten-level disparity between PC's and their enemies would still be pretty much un-workable in 4e (though for different reasons).
But this is a pretty good example of how 3e, much like the older games where combat wasn't necessarily something you wanted to get into, inherited the "Boy Scout Problem": You must always be prepared for everything. Back in 1e, the jokes with the 11-foot poles and hireling armada and the like poked fun at this. 3e inherited many of 1e's more deadly motifs, though the joke was usually more along the lines of "never camp within 20' of each other out in the open," but the sentiment was the same. If you don't prepare, you will be dead, and the DM can always kill you if he wants. For certain styles of game, this is totally OK, even great, since combat isn't their big thing.
I think for most, though, it tends to be frustrating. Which is why I'd advocate swingy without being binary. Even a rule like "If you are reduced to 0 hp, you have one round before you go unconscious" would ameliorate this almost entirely. Push it to the edge, but not over. Don't let one round decide the combat, give the other side time to respond, but don't be afraid to FORCE the other side to respond. IMXP, in 4e, the decisions don't carry much weight or importance, since they largely do the same thing, and this is mostly because 4e wants to jettison most of the swing. Which can be an issue if I'm looking for dramatic tension on every round (which is something I'm looking for more often if my combats tend to be fewer, and more narrative, rather than more, and more simulation).