The uploader will be banned and their content removed, and hopefully will have to deal with the police. If a PeerTube instance is too laxist about its moderation policy, other instances will stop federating (federation is opt-in and decided by the admins).

or copyrighted material. this could open up peers to civil infringement suits from the MAFIAA which could have a chilling effect on new participants.

If the country the instance is hosted in has illegalized child porn (japan for example allows some forms of child porn and it's normal to consume those), then the instance will likely take it down fairly quickly. If they don't they might be unfederated by other instances that definitely don't want this.

It's censorship resistant because each instance is an island and yet every island is connected. As long as someone is willing to host your content, people on the fediverse can consume it one way or another.

The person responsible for that PeerTube instance would have to remove it, and hopefully ban the user and report them to the authorities.