I find cross pots very annoying.
They divide the discussion, there can be a single cross post with five different children, and each one has one or two comments. That's bad for the Lemmy community
They fill up the feed with repetitive content, which can go from oh that was interesting to frustration seeing something over and over again.
They can be used to top post somebody's content to a different community, stealing the interactivity of the original poster. This is especially true for people trying to grow a small community and posting content
Where I have seen crossposts being useful is when two communities that are diametrically opposed when to talk about the same thing