Say you have two pieces of fish in front of you. Answering the question "which one is more radioactive" requires more than holding a device over each for a couple minutes. You have to think of the flux, the surface area of the fish visible to the detector, the orientation of the detector, any background sources, the mass of each piece and most importantly the distance between the fish and the detector. That cannot be built into a hand-held consumer product. Absent that, these devices will only scare people.
Note the pic in the OP showing the detector plugged in via an extension cable. I'd bet that they moved it around until magically it's measurement lined up with the other device.
Also, low levels of radiation are nothing to be concerned with. The linear no-threshold model (the direct relationship between radiation and cancer) is no longer considered appropriate when discussing very low levels.
> Measurement error – <30% within a given deviation between
For many people even 50% error is acceptable. It is just to check if house or hotel is safe. I live at place where radon, radioactive ashes, low concentrations of uran.. are credible danger.
There is plenty of evidence that the stress created by a device like this, by an obsession with infinitesimal amounts of background radiation, does far more harm. I see no net health benefit from such products.
Fyi, a 25% error rate detector can be build using a soup can and some string. (Not a joke. This thing actually works.)
Considering how inexpensive and reliable radon test kits are, it may be advisable to stick to that.
I am surprised to see that they list the operating temperature from 10 to 40c - I wonder why it stops working outside those bounds or how off those bounds are. It would love to see someone to order a few of these and review them in all sorts of different tests.