Next to Morrowind, I pretty much hate Oblivion. I can't put my finger on any singular thing to blame for why I hate Oblivion. Not a fan of blame altogether. But I'll say there's an interaction between a melee-stealth character, scaling, and Clanfears that made it a very unpleasant experience. Clanfears being the biggest offender in the creature category, but most of them share it to some degree.
The result of the interaction of factors is that a melee stealth character spends a good portion of the game getting absolutely tossed. As any stealth specializing character will experience, your stealth will be probably the thing that goes up the most at the begining of the game. Which, after a few levels, leaves you absolutely boned for the combat that results after your first strike. Even if they're major skills, your endurance and ability with your weapon have not gone up to the same relative degree that your enemies endurance and strength has. So, your tactics have to drastically change for what were previously mundane tasks. It's a difficulty increase for no sensible reason. At this point you adapt by either switching to the bow (which you may be able to use to either kite, get multiple sneak attacks) or using massive amounts of poison. If you encounter a Clanfear, the necessity of using a bow is critical. But also an exercise in futility because there will be a relatively distinct window of your gameplay experience where even causing a Clanfear to look at your fragile body will cause you to automatically die. When you enter this window, you need to change things.
I fully accept my own fault in things developing to that point every time I play. It's a choice of playstyle on my part that led me to such a situation. I never was overly interested in any particular endurance skill. But, I also recognize Bethesda's fault for implementing a design where early decisions can result in later events going so terribly pear shaped without warning and without giving reasonable methods for rectifying the situation.
But what's even weirder is how inconsistent this difficulty is. Early on things are well balanced. You can sometimes sneak up to someone, sometimes get the cheap shot in, then wittle them down normally. Then it transitions to a period where you can sometimes sneak up on someone, sometimes get the cheap shot in, but regardless of if you did those two previous things or not, it's time to start kiting while emptying your bag of every potion you could possibly managed to stock up in a "and the kitchen sink" manuever. As your sneak skill increases, you transition to yet another period for the rest of the game where nobody ever notices you. Not even while you're stabbing them repeatedly in the back. It all forms a retarded bell curve difficulty. It should be noted that in this later period where you stab someone repeatedly in the back that, if they did notice you stabbing them 10 times for the highest sneak attack multiplier, you would be truly and utterly boned.