USD forks are stiffer for the same weight as the larger OD members are in the most stressed position, the top. The lower sliders can get thinner, thus lighter and still be satisfactory.
As the forks moving/oscillating structure is now lighter, unsprung weight is reduced, the suspension becomes more compliant. The front wheel has an easier time following the ground.
As the seals are closer to the ground, there's more opportunity for grit damage. Usually plastic shields are provided to mitigate damage. As the seals are "at the bottom of the oil" a leak can result in dumping most of the damping oil. On the street, a cleaner environment, this seems less an issue than for trail use.
Actual performance is determined by the money, materials & engineering invested. In theory they are inherently a better design.