That pretty well is the same experience we have here between my Davis, and my neighbor's Tempest.
I think from an electronic sensor perspective, temperature is easy, and from there it starts to get more difficult, and the rain and wind speed is not easy. Accurate weather stations without moving anemometers and rain gauges seem to be extremely pricey when I have looked at them, and they do the non-contact measurements differently from the Tempest methods.
I do hope that Tempest succeeds on improving their accuracy. Given that by now they would appear to have more than enough data at this point in time to improve their software/firmware, I suspect that they have some hardware deficiencies/impediment(s) in the existing versions. I think that sites like
@Popgadget's would be ideal for software improvements. I think that does beg the question why they haven't rolled out a new generation of more accurate sensors.
All the best,
Peter