To each his own opinion,
To me a successful unique is something that is impossible to obtain otherwise, new mechanics, or are build enabling. There’s a very important aspect of equipping an unique: you lose all the possible affixes, and the possible item base with its implicits in that slot. That on itself is a detriment for any unique.
Let me talk about one item that has been brought to the conversation: Suloron’s Step.
Suloron’s Step, looks promising, that’s a huge crit multiplier, can have decent movement speed due to that big movement affix that could match even be a tiny bit better than boots+ movement suffix, but the -75% physical just kills the item, because:
This is a tank game, and we cannot afford not only losing the inhered flexibility you lose using a unique, losing the possibility of a good base with a good implicit (fire/cold res), plus losing three affixes that usually are hybrid health, vitality and some resistance or percent dodge. You have to already to cover for that, then add two T5 phys res affixes in other places to cover for the phys res loss.
Also, any crit build has his ways to get crit multi from other sources, not only items, and any final crit build usually have about 400% crit multi already, and ways to be sure you crit always, at least when it matters.
So, you win: Up to 105% crit multi (worth 4x T5 crit multi in relic), some minor movement speed with a good roll (3-5%?).
You lose: Boots Implicit (could be a T5 res), Up to 8 vitality, up to 65 health and 5% health, one T5 suffix for anything, two T5 phys res suffixes in other places.
So, is a bad designed item, because is not aware of the tank meta, how you already have tons of crit multi, and you lose more value that you win (arguably you trade off 4x T5 offense with 5x T5 flexible value/defense + boots Implicit)
If you remove the -75% phys res, would be, all things considered, just trading defensive value and flexibility for offense value, and would be a great item to have around if you are going crit and you can afford the loss of flexibility.
Then we can consider exalted affixes and LP, but for me those shouldn’t fix an item usability. I consider those more of an endgame min-max and win-more mechanics. The item becomes useable from a pure value perspective if you have a good 2LP one.
By this metric of potential value loss and lack of flexibility, half of the designed uniques are just not worth the slot unless you get LP (and sometimes only 3-4LP make the item worth the effort to slot it in).
Uniques should be something worth considering when building your character without LP, when all their properties are useful for your build, is just not the case.