One thing to keep in mind is that you can't loot HP. Upping their toughness by giving them more health instead of legendary daedric armor achieves roughly the same effect, but doesn't break the game's economy or make smithing obsolete by flooding you with top notch gear.
Weapon damage though is another matter, but there are other ways to improve NPC damage besides giving them awesome weapons. I haven't looked at any of the NPCs in an editor yet, but I'm guessing that even if their weapon skills scale up as they level, they probably don't get any perks. With 5 ranks of damage increase, you'll get 200% damage, which, multiplied by the 50% increase you get from 100 weapons skill, means 300% total damage increase. NPCs are stuck with the 50% from skill alone. So adding perks would help keep NPCs a bit more powerful later in the game.
Smithing can roughly double weapon damage, and enchanting can almost triple it, so you're still going to be looking at a discrepancy, but it would still be a big step up.
I have been single hit to kill with an bandit with an orcish bow, level 46 with 250 hp and 300 in armor, also been killed in one hit from an dragon, lots of other enemies like bears do serious damage and can easy kill you, I'm not so concerned about magic attacks as I have potions who makes me resist most of the effect but without them you can easy bee killed at all levels, playing on hard who work well as I'm able to do plenty of damage and expect the enemy to do damage to, yes most of the enemies are weak but some are dangerous, you just has to know who of the forsworn who are the briarheart

This is a far step from earlier games, Daggerfall and Morrowind lacked high level enemies. In Oblivion enemies at higher level just got tons of health but was not dangerous as their damage was to low, they just took a long time to kill.
Fallout 3 was better but enemies before broken steel was not very dangerous at high level with the exception of the rocket launchers.
Obviously high end npc get some sort of damage buff in Skyrim.