There is a lot of crap about but some of it works - to a degree.
My company used to import a 'restorer' that did work if you used it in accordance with it's capabilities but the 'snake oil' label killed it in the market.
They will not work if your engine is **acked , but will deposit PTFE or similar compounds to reduce leaks, tighten clearances etc.
I have used them in the Missus car to good effect.
I think the NASA claims are a little stretching the point, but the core technology involved in the science of 'tribology' did come from the space programme, just the yanks have more elastic advertising rules than us.
But, lets face it , pouring a can of stuff into a rocker box is no where near as much fun as spending a couple of weekends and a few hundred notes stripping down and rebuilding a motor !!