There was a bug in Safari's registerContentScripts that was only just recently fixed in 18.6. uBOL needs that bugfix.

There was also bugs in the declarative net request implementation that was also just recently fixed - quite a few bugs have been holding uBlock back. The reason why uBlock hadn't come sooner was because DNR hadn't been implemented which would have required the developer to go out of their way to specifically re-write uBlock Origin to work with Apple's content blocking API but now Apple has implemented DNR it should open up the possibility for more choice in the content blocking extensions market.