@Ursus Hi - Cannot help you over Homekit but I can say that the tado smart thermostats I have are within 0.5 of a percentage point when measured against recently calibrated equipment.
Common issue recently, no official statement as of yet :/
Maybe the issue isn't that the values are incorrect but that the HomeKit values are not updated as often (edit: I just realised that the OP already made this assumption too). I am running both the API plugin and the HomeKit in parallel on Homeassistant and I am seeing far less updates on the HomeKit integration than using the API. Sometimes updates are 8+ hours apart. The official documentation states that humidity is updated at least every 15 minutes or if the humidity changes by at least 5% since the last update. This does not seem to be the case with the HomeKit integration which is really annoying as it makes the humidity value unusable when using HomeKit.
Is there anyone from Tado able to confirm what I am seeing?