Well, it is extremely easy for law enforcement to compromise the Tor network. All they need to do is control the vast majority of Tor nodes, which they can easily do.
Just control the Tor exit nodes. Monitor all the traffic at the ISP level. Since most cp downloads are from servers located on the clearweb, they need to use the Tor exit nodes. Just monitor how much traffic is exiting the Tor network. Monitor everyone accessing Tor at specific time frames. And Tor users are easily traced.
As for people accessing Tor hidden services, the police can control the Tor entry nodes. And if those illegal onion sites are honeypots run by the police, they can check whoever is logged in at specific time frames. And monitor who is connecting to Tor at specific time frames to figure out who is logging into an illegal onion site.