If you happen to find the wrong champ and you are on an edge difficulty for your toon, you run into severe problems. ToEE on EE champ archers hitting for 2k damage AFTER PRR is applied? Happens. Archon quests champ Red Abishai oneshoting a MRR 100 toon? Has happened. Not too often, actually. But happens.
Since champs seem to be very random, you may meet all and everything from harmless meatbag (or even the strange harmless champ on half hit points - my harmless favorite) to one-shot-killers. Even if those are rare, thank god.