When uBlock first came onto the scene, its main selling point was that it had vastly better performance than Adblock when using filter sets that had lots of block rules based on CSS selectors rather than just URL regexes. And third-party filterset subscriptions that made heavy use of CSS-based element blocking rules had become quite popular by then. So an API that only provides for URL blocking is missing almost the entire point of uBlock.
(I wasn't using any third-party filtersets at the time, so I found uBlock unimpressive and stuck with Adblock Plus for quite a while because I found its UI better for curating my own blocking rules.)
I don’t know how uBlock handles CSS rules, but you can just add your stylesheets or add your JavaScript to the page, it should be enough and as performant as possible.
AFAIK there are esoteric features like DNS filtering which is not possible with declarative API. But I’m not sure if it’s used widely, I never saw it.
(I wasn't using any third-party filtersets at the time, so I found uBlock unimpressive and stuck with Adblock Plus for quite a while because I found its UI better for curating my own blocking rules.)