While I haven't experienced USWDS deeply, that's kind of a problem in itself. A good design that doesn't get deployed is just that - a design. Gov.UK might not be pretty but it's consistent across different ministries, departments and instrumentalities which helps tremendously, but US government websites, even if restricted to the federal level, is a mess that I would prefer to hold my tongue at the inconsistency. What is GAO's plans to deploy them at least at a federal level? Is there even a plan? By the way, it's not solely Gov.UK which has this level of consistency (Singapore comes to my mind) so it's not a unique property.
Gov UK design system is poorly usable. From color use to disorganized, scattered layout; it’s put together with haste and not much thought.
What it’s good at is looking sexy and minimal which captivates a lot of people.