I'd look into/think about: what happens when the kit does not detect something that then causes harm to someone.

Is that likely? Could it happen? What might happen legally?

I'd likely consider buying something like that, though, if it were cheap.

Something that would be positive: testing for 'unknown unknowns' - people are becoming more conscious of things like radon, but if this test were likely to pick up a wide range of random stuff, it'd be more useful.