Excellent monitor
Could the app use some work? Yep, for example it could save / show the data in a format that includes time and date to make it more legible to humans. At the same time, it’s biggest use is determining whether a space is safe or not. For that, it works exceedingly well (with the usual time delay of any sensor) and mitigation efforts should show up decisively, if effective.
I used RadonEye to ensure that the repairs we did to the basement made effective reductions to absolute as well a average radon levels. For that task, it is perfect.