There's nothing to clog up. The drain is a normal drain; the innovation is that the cold water source pipe winds around the drain, transferring some of the heat from the outgoing water to the incoming.

Heat transfer requires time, which is why I've always assumed that such a system would have to use coils somewhere.

In the system under discussion, all the heat-transferring parts had better be metal, and it won't be worth a damn except when handling a hot shower or a draining bathtub.