You can read here: https://github.com/uBlockOrigin/uBOL-home/wiki/Frequently-asked-questions-(FAQ)#is-ubo-lite-a-bad-faith-attempt-at-converting-ubo-to-mv3

My goal with uBO Lite is a reliable and efficient MV3-compliant content blocker, leveraging the filter lists used by uBO. I am satisfied that I fulfilled that goal by ensuring uBO Lite was entirely declarative -- though at the cost of limitations beyond those intrinsic to MV3.

Reliability: Being entirely declarative, uBO Lite's service worker is not required to initiate DNR-, cosmetic-, or scriptlet-based filtering in a timely manner.

As a result, uBO Lite reliably filters at browser launch, or when navigating to new webpages while its service worker is suspended. This can't be achieved without uBO Lite's declarative approach.

The choice of being entirely declarative for the sake of reliability and efficiency meant sacrificing being able to create custom filters, hence the "Lite" in uBO Lite: it's not meant as an MV3-compliant version of uBO, it's meant as a reliable Lite version of uBO, suitable for those who used uBO in an install-and-forget manner.

Example:

Pity you cant as there's no filterlist I'm aware of that can stop the artificial delays on youtube which can be blocked on UBO with ||googlevideo.com/videoplayback$xhr,3p,method=get,domain=www.youtube.com
having to put up with the constant delays on youtube because that filter cant be added is a real pity.