I usually wait a little while for the water temperature to stabilize, before getting in. That transient period will be much longer with this gadget. - time it

I usually wait a little while for the water temperature to stabilize, before getting in. That transient period will be much longer with this gadget. - time it takes for the heat exchanger (gadget) to warm up due to shower drain water. - now the shower water temperature has changed (increased), so you need to adjust the valves. - another transient time for the heat exchanger to warm up, etc. Depending on the design, you might find yourself wasting a lot more water, or jumping around in the shower as the temperature keeps changing in this slow control loop.