It often works, but sometimes fails in very quiet ways. https://pawb.social/post/8422352 and https://yiffit.net/post/8267386 are the same mealtime video post (Technology connections kettle), but very different comment counts. I usually see it as posts or communities that just stop showing up even though nothing changes. The other day I played with the stories community here from a remote community, but nothing showed up until I made a comment on a post that didn't show up initially.
The way the instances decide to federate or defederate explains why you can see the counts and comments change between instances.
If you were to, for example, log into Lemmy.World you wouldn't see anything from Hexbear, but here on Yiffit you can, because World is defeated from Hexbear, but Yiffit is not.
I think, but not sure, so long as you are federated with the community you can see comments and posts from everyone, its just the communities hosted on the defederated instance that you can't see.
I'm pretty sure it extends to users as well. It's like a block of everything from a particular instance/domain. The blocked users can still see the post and interact with it, but anyone on the instance that blocked them won't see their content.
I'll have to switch to another instance next time I see comments from instances I know are defederated from the account I log into to be certain, though.