Unlike Bootstrap (and Foundation), UIKit has no statement about the accessibility of what it provides. Some of its JavaScript includes ARIA attributes but who knows if they did it correctly; adding ARIA wrong can sometimes make sites less accessible than if it wasn't used at all.
UIKit has many CSS classes with hover states but not focus states and it's not easy to add them to classes like .uk-hidden because it's not a direct declaration but relies on a parent class. It lacks an equivalent to Bootstrap's .sr-only (visually hide but expose content to screen readers); you can always add such a class yourself but it makes the UIKit framework incomplete. UIKit makes it harder to make a site accessible than if UIKit was not used.
CSS Frameworks are supposed to make the job easier by taking care of problems you're not even aware of; UIKit fails to do that in this respect.
UIKit has many CSS classes with hover states but not focus states and it's not easy to add them to classes like .uk-hidden because it's not a direct declaration but relies on a parent class. It lacks an equivalent to Bootstrap's .sr-only (visually hide but expose content to screen readers); you can always add such a class yourself but it makes the UIKit framework incomplete. UIKit makes it harder to make a site accessible than if UIKit was not used.
CSS Frameworks are supposed to make the job easier by taking care of problems you're not even aware of; UIKit fails to do that in this respect.