It’ll make it a lot more likely that YouTube ads will get through because MV3 limits the block list size to a fraction of the size normally used by uBO and also disallows external/live updates to the block list, instead forcing the rules to be baked into the extension. Meaning an update to the blocking rules could take a week of extension review time to go through. I heard that the YouTube ad blocking rules can update multiple times a day so this would easily allow Google to update their ad code before approving updates to ad blockers, allowing them to always stay ahead.

So it might not outright break it, but some rules will have to be left off so it seems like it’ll be a dice roll if you get an ad where the blocking rule had to be left off to fit Google’s block list limit or the rule you have is stale because it took a couple weeks for the extension update to be approved on the extension store.