That points to a lack of QA on your part and, I think, it is fair for a reviewer to point out.
Even if you have an exemplary warranty process and easy instructions, that's still a hassle. Not everyone has the confidence or the time to repair simple things.
As for the objective/subjective nature of reviews. Are your customers buying air monitors for their 100% precision or for "entertainment" purposes / lifestyle factors?
I have a cheap Awair air monitor. I have no idea if it is accurate - but it charges by USB-C and has an inconspicuous display. That's what I wanted it for.
It is perfectly fair for a reviewer to point out their personal preferences on something like this. They aren't a government testing lab.