The issue is NPC power vs player power, and why it tapers off so rapidly from imbalanced abilities and rudimentary NPC design. One of the biggest is readily seen on NPCs that are of the same humanoid races as the player, and are thus not considered under the "creature" type.
Aside from obvious things like sneak detection Ai being blind/deaf and general exploitable/dumb Ai behaviors, they appear to scale in a way, that directly increases their outgoing damage, and inflates hit points to compensate for poor equipment. This isn't inherently bad exactly, but it is the lazy way out, and is made glaring by most NPCs not having any perks, upgraded equipment, or skills. This is also an obvious cause for player abilities being overpowered.
It is the fact that NPCs cannot use them, and are thus essentially on a different ruleset from the player, when Ideally, they ought to be constrained to the same rules we players abide by, and benefit from...that is an issue.
Not only has this likely given the developers a perceived excuse to not have to balance perks and equipment levels, under the assumption NPCs cannot wield them at all against players, but it also leaves NPCs in a constant state of playing catch up through brute force due to their lack of ability...which obviously is not working.
It becomes more pronounced with "creature" type enemies, as they are even more divorced from player mechanics. They have 0 armor value, no damage penetration, perks etc.
The result of said issues is thus: Enemies are so basic and pathetic, that many perks aren’t even worth taking because they have no use, or are blatantly broken when put to use against the opposition due to an apparent lack of playtesting and scrutinizing of such abilities beyond simply seeing if they work.
Examples:
Bladesman only works off of base damage of the weapon, which is a fraction of total damage at high levels. For example using a Legendary Daedric Sword with 100 One-handed and 5/5 Armsman you will deliver 72 damage per hit. However base damage for a Daedric Sword is only 14, so 3/3 Bladesman perk will have a 20% chance to cause 7 additional damage.
The bleeding effect of Hack and Slash depends on weapon material and skill level and lasts six seconds. However even a Daedric weapon used with level 3 of Hack and Slash will cause only 18 bleeding damage. While multiple bleeding effects stack, by the time you apply several of them the enemy will most likely already be dead due to our weapon's raw damage.
The effective value of 100 sneak skill is apparently so high, that increasing it further with enchanted equipment, causes the hidden value to roll over past 100 to start over again despite still showing 100 sneak.
Shadow Warrior, does not do anything close to what its description says. It applies a faux invisibility type effect that does not really draw off distant enemies, and in fact allows you instead to crouch and strike an enemy repeatedly until you get a sneak attack after already being detected....throwing the need for precise sneaking out of the window.
Mages with the impact and relevant elemental perks can essentially staggerlock every enemy in the game that can be staggered to the point where they cannot even fight back at all due to the issues with stagger in general, that being that there are not stagger thresholds, or stagger immunity periods preventing spammability.
The armor-ignoring effect of Bone Breaker is almost completely irrelevant because most enemies are lightly armored. Dragons and most monsters have high health and 0 armor, and even heavily armored humanoid enemies most likely have no armor skill, no armor perks and no armor upgrades, so an opponent clad full in Daedric Armor will have 49+18+18+23=108 armor rating, which gives them negligible damage reduction.
Player mages, that focus on pure destruction are far weaker than NPC mages that do the same. If players and NPCs followed the same rules, this would have been noticed unless it is fully intentional.
Sneak attacks do ridiculous damage, due to the above...but the damage itself obviously does not really need a significant nerf as that is not exactly the real problem. The problem with sneak attacks is that there is no noticeable amount of damage reduction going on to mitigate the high damage values against enemy hit points to justify the damage being so high in the first place.
The marked for death and similar weakness to resist/defense type abilities, are supremely effective against dragons and creature type enemies, due to them having essentially 0 armor to begin with. You are reducing their defense below 0! The old stacking bug with these abilities has also migrated. Using two level 1 marked for death shouts against one enemy for example, is better than using level 2 marked for death!
Dragonrend, at all 3 levels, comes back before the duration expires...making it 100% sustainable. Rendering dragons even more useless than they already are as "powerful boss enemies".
In short, I am sorry to say that this level of enemy design/mechanics work is incredibly lazy for a game that has been under development for at least 3 years. Considering we will typically spend a majority of our time dealing with the combat mechanics and enemies, I find its current sad state inexcusable.
This is only one of the issues I have with Skyrim, but that's not to say that I hate the game, far from it. But it does sadden me to see something that could in many ways, have been so great....diminished by things that were simply not given as much care and diligence as they should have gotten.