Uniques have multiple issues, first being terrible drop rates. Second is their terrible scaling.
First point:
Drop rates are so low that you're lucky to see 1-2 a day of playing. This would not be so bad if they were amazing good... but they are for the most part extremely underwhelming. Also, consider that since there are no classes, there is no way to drop gear more suited to your build, so you could get ANY of the 40 or so unique items, to which only maybe 1 or 2 will be decent for you. Gearing is very quick in this game as well, so you constantly find little upgrades as you progress through the end game. This means any uniques you found that you might like, quickly fall behind regular gear you are gathering.

Second Point:
Unique gear does not seem to scale are higher level. Example:
Frayed Combat Cleats - Rogue Boots - Level 103
(No Base stat)
-31 to All Resistances Score
+1 Stamina Point
+39 Agility
+11% Movespeed

versus my:
Imperious Sollerets (Rare) - Heavy Boots - level 73
+19 Toughness (Base stat)
+30% Material Damage
+1 Stamina Point
+22% Movespeed
+102% All Resistances Score on this Item

My rare boots 30 levels lower blows this unique out of the water in every possible way AND doesn't have a negative aspect to it... This happens because despite the level being 103 on the boots, its actually scaled to its equipable level which is 35. This means Uniques will likely never be good for anyone aside those running through campaign... to which they are so rare they might do entire campaign without seeing a single one... Uniques really need a re-design.