The only detrimental thing I can think of is its going to mess with your ignition timing a little because it does alter the coolant temp signal that the ECU sees.
I know this spoofer is smart though. It waits until the coolant is 116F until it starts working to avoid cold engine enrichment running problems. There was also something else about a check engine light that was programmed around when I was reading the install manual which also had revision notes on it.
There is tons more info on the spoofer here:
A Prius ECT Spoofer MCU Controlled | PriusChat
The same general design could be used for any normal car though.