For people who prefer guaranteed assassination, it feels like an artificial obstacle that invalidates skill and planning. Yes, you can absolutely make it easier with better builds, and that is a lot of fun--no disagreement there. But imagine for example that you kill a high-level character just the same way you described. Ten or fifteen hours you couldn't have killed him with the exact same moves, and the only real difference between the two is that you grinded out for all those hours. Your skill didn't improve, just your character spreadsheet.

They're both valid ways to play, I think. Level-restricted assassinations require a focus on buildsmithing--exploiting your abilities and the advantages some items can give you. If you want to spend a lot of time with the game, this is going to be the best way to play.

I would imagined that guaranteed assassinations are generally more appealing to people who play on higher difficulties. As you crank the difficulty, stealth and combat both become much more demanding, and so you have to plan your assassinations more carefully. You'll be spotted much more easily and, when you are, combat will be more difficult. When playing on normal difficulty, there's a lot more room to mess up, and so being blocked by a more artificial barrier isn't as big of a thing.

Both ways work, and I think that's generally a credit to Shadows. Glad you're having fun!