Ok, so it seams random loot is the only advantage. What about my drawbacks? Any opinions?

I think a system like this has no place in a static curated world.

I don't like random loot. If I wanted random loot I would play Diablo or WOW.

What I meant with the level dependencies of unique weapons is exactly what you described. They are bound to a level. If you exceed this level the unique weapon gets replaced by a common weapon (of a higher level) which does more damage. So the "unique" weapon becomes worthless. And if I somehow encounter a boss a little bit to late (one level higher) I get rewarded with something that may last a whole level till I need to replace it.

Another example why this system is bad: Imagine 2 characters, both level 3. The warrior (strength 10) is equipped with level 1 stuff, the mage/non fighter whatever (strength 5, dex 5) has level 3 stuff, armor and weapon.

Who will win a melee fight?