I don't think there is enough information out there about the effects of hydrogen embrittlement and hydrogen blistering of steels from this "cleaning" technology.
Here is a review of the issues with hydrogen in an ICE environment:
https://www.google.com/url?sa=t&rct=...rTHWR2AxWHvMMG
And the concern with Hydrogen and metals:
https://aip.scitation.org/doi/10.1063/5.0012851
I've read on some HHO sites not to worry about hydrogen embrittlement as they are supplying diatomic H2, not atomic hydrogen. This is like saying don't worry about CO2 emissions, we're just supplying the carbon source and the diatomic oxygen. It's the chemical adsorption of hydrogen onto the Ironoxide that results in the formation of atomic hydrogen. Your car vibrates a lot while in motion and whilst this cleaning might be a good thing for deposits it might not be the best thing for longevity of your engine, so there is that.