Thanks for linking. So this post talks primarily about missing support for noop ("no operation") rules. Jumping into the uBlock docs, they say:

>A request can be blocked (block), allowed (allow), or ignored (noop). A noop rule will cause matching network requests to be ignored by the dynamic filtering engine, but those ignored network requests will still be subjected to static filtering (filter lists).

So the missing limitation in MV3 right now seems to be that things can be blocked or allowed by dynamic rules, but not ignored such that static rules (be they from a list or the user) can take over.

I'm not sure what use case that has exactly, but I'm guessing that could be a problem if a user wants to override an existing filter with a custom, dynamic rule. Is that the right idea?

The main issue is that both conditions and actions are limited. At the very least they would need to allow registering self-contained functions as conditions and actions, nothing less will allow extensions to have proper control over requests.