One of the major issues is replication and propagation of illegal material. Because of the way that content is mirrored and replicated across the fediverse, attacks that flood communities with things like CSAM inevitably find their way to other federated sites due to the interconnectedness of the fediverse.
The only response currently to dealing with these types of attacks, even if they're not directed at you, is to generally defederate with the instance being attacked. This means whoever was attacking the site with CSAM has won, because they successfully made it so that the community becomes disjointed and disconnected from the rest of the fediverse with the hopes that it will die.