The way Bethesda handles difficulty is pretty lazy and consists of either giving enemies more health or letting the player increase "difficulty" by setting a damage multiplier. Perks aren't distributed sensibly according to enemy level, and instead they're usually given damage multiplier perks. Enemy AI is similarly barebones and mostly controls movement patterns or how often they use certain attacks. Most abilities, potions, scrolls, staves, and spells the player has access to don't work at all for NPCs, like racials, bound weapons (most enemies that use them just have a similar looking weapon placed in their inventory), runes, area spells like wall of flames, or any illusion spells. Not to mention for potions and poisons they only know how to use healing or restoration potions and even then only sparingly.
You can use scripts like NPC Stat Rescaler to have NPC attributes scale similar to player attributes instead of getting massive health and magicka pools (or you could have them scale any way you want, really), and ASIS to distribute level-appropriate perks and spells, but problems with enemy AI remain and there aren't any really good solutions out there. Some try to use the tools Skyrim has available, like Wildcat, while others use custom plugins to add things like roll dodging, parrying, and poise, but no matter which one you use you'll never see enemies taking advantage of terrain or using all the tools the player can use.
All of this means combat doesn't require much thought and is down to battles of attrition, just whittling enemies down with the same weapon or spell every single fight. Weaknesses aren't worth taking advantage of and resistances can be mostly ignored, and similarly enemies almost never switch up their approach in fighting you. There's a mod called Know Your Enemy that gives creatures and armors resistances and weaknesses which is a step in the right direction, especially when paired with other mods to make NPCs more challenging, but not enough to make combat really interesting as it would require a significant overhaul of NPC behavior and capabilities.