One thing I miss in activitypub is a verification system. This is most important for deletion. For example, when something is deleted on one instance, there's no way of knowing for sure if it will be deleted from other instances as well. An instance being down or malfunctioning can cause deletion to fail with no way to know* and try again for op.
*To know one would have to check every single subscribing instance. Trying to delete an already deleted item is obviously impossible