I have been comparing HYT221 and SHT21 for about 6 months now. Lately, HYT221 started to have significant wet bias, if the real humidity level is above 70%. This means that if the near official station and SHT21 think the humidity is about 80%, HYT221 can show that RH is more than 90%. However, if the humidity is less than 70%, both SHT21 and HYT221 have similar readings. Actually, HYT221 is very close to the official station, if the humidity is around 20%-40%, while SHT21 has a slight wet bias all the time.
It is worth of mentioning that it has not rained or there has not been fog/frost for weeks now. Both sensors are now installed in the same radiation shield and HYT221 has no more a DIY plastic splash cover (but of course it has its own yellowish filter). I have no idea why HYT221 has started to act like this. In the winter, it performed very well. Also, HYT221 has +0.4 Celsius warm bias at cold temperatures (if outdoor temperature is less than 0 Celsius) compared to the SHT21. I trust SHT21 temperatures more, since I have other thermometers as well and they agree with SHT21.
In conclusion, SHT31 and HYT221 both failed within one winter. SHT31 has near constant -10% dry bias, HYT221 has +10% wet-bias at high humidity levels and SHT21 has not-so-bad wet-bias. I really consider buying a Vaisala sensor to stop this nightmare...