I rely mostly on the discussion around the implications for ublock origin when looking at manifest v3, because gorhill seems to be engaging with it in good faith.
Browsing the issue about it (https://github.com/uBlockOrigin/uBlock-issues/issues/338), it seems that manifest v3 blocks quite a lot of stuff out of the box. The fundamental issue seems to be that by mv3 prevents you from running arbitrary code to do filtering. This is concerning because it means that if sites / ad networks start coming up with ways to serve ads which can't be blocked by DeclarativeNetRequest (the issue lists a few), then ublock origin won't be able to block those ads. The way it's framed in the issue is "DNR is an obstacle to innovation."
There is currently a workaround in "ublock origin lite" (the mv3-compliant version) which lets you opt into regular ublock-origin-style filtering on a per-site basis. Even if this workaround isn't removed, it still means ... opting in to full filtering on a per-site basis, and acknowledging a browser warning each time.
MV3 clearly puts control of how ad-blocking works back into the hands of the company who supplies the browser and makes its money by serving ads.
I think it's worth noting that MV3 also solves problems. Like, the fact that an extension can do arbitrary things is obviously very dangerous. The ability to intercept arbitrary API calls and scripts is very powerful.
So the goal is to remove that power, so that it can not be abused. Then the question is "and how do we solve the legitimate use cases?" - hence the years of working with people like Gorhill to help build a better API to serve the ad blocking use case.
MV3 was delayed because, despite the numerous changes made to MV3 to satisfy these use cases, it's clear that there needs to be more work to do so.
Browsing the issue about it (https://github.com/uBlockOrigin/uBlock-issues/issues/338), it seems that manifest v3 blocks quite a lot of stuff out of the box. The fundamental issue seems to be that by mv3 prevents you from running arbitrary code to do filtering. This is concerning because it means that if sites / ad networks start coming up with ways to serve ads which can't be blocked by DeclarativeNetRequest (the issue lists a few), then ublock origin won't be able to block those ads. The way it's framed in the issue is "DNR is an obstacle to innovation."
There is currently a workaround in "ublock origin lite" (the mv3-compliant version) which lets you opt into regular ublock-origin-style filtering on a per-site basis. Even if this workaround isn't removed, it still means ... opting in to full filtering on a per-site basis, and acknowledging a browser warning each time.
MV3 clearly puts control of how ad-blocking works back into the hands of the company who supplies the browser and makes its money by serving ads.