I remember reading on a plumbing blog (or YouTube or something) about someone who installed one of these - high end house, massive shower, owner who liked to take hour long hot showers or something.
In that case, it paid for itself pretty quickly.
For "normal use cases" the cost savings over time just isn't there, when you compare the added complexity. You're better off saving heat with on-demand heaters or HVAC recirculators, etc.
Places where it could make sense already use them (I believe car washes recycle water and heat in some locations).
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.
In that case, it paid for itself pretty quickly.
For "normal use cases" the cost savings over time just isn't there, when you compare the added complexity. You're better off saving heat with on-demand heaters or HVAC recirculators, etc.
Places where it could make sense already use them (I believe car washes recycle water and heat in some locations).