The only time I really was truly screwed was when the companions sent me to a falmer cave on one of the first quests and it was my legendary playthrough. Neddless to say I may have lowered the difficulty. There was one dragon priest that was pathetic, it was the one on the winterhold questline. I had a ton of magic but still used a sword so I let him use the staff a while before I ended his pathetic life.
I know how you felt... I got sent to Shimmermist at level ~10, on Expert difficulty with a populated dungeons mod (= more Falmer/Draugr/bandits/etc.) installed. After 1,5 hours I made it to the boss room, which took an additional 30 minutes.
Morokei has never given me any real trouble, possibly because I never, ever play a magic-only character. He really is a pushover when you use a combination of archery and melee (plus Shouts).
Only the shock Dragon Priests have ever given me a truly difficult time. Zahkriisos in particular is INSANE when you don't have Become Ethereal or resistances. And he's at the end of such a long and (at times) tedious dungeon, too.
The hardest enemy I have
ever faced is Vulom in the Wyrmstooth mod. While he didn't do much damage to me, it took me half an hour to figure out how to effectively deal damage to him.
New lands mods aside, the hardest enemy I've fought so far is Alduin. I'm using two dragon mods, both of which make him much stronger than a Legendary Dragon. The one time I got to "Alduin's Bane" he totally and utterly annihilated me within five minutes. I was level 50+ with legendary Stalhrim equipment, a ton of resists and SkyRe's higher damage output.