Out of the box, ALL temperature sensors are inaccurate. If the device hasn't had a multi-point calibration then your actaul values are meaningless in any case, be it a thermocouple, NTC thermistor or PT100. In the absence of proper calibration, what matters is repeatability and the difference in numbers you are getting - if you find a batch is too fruity/estery, after controlling fermentation temps, then you know you just need a lower number than the one you had; the actual number itself doesn't matter.
I recently brought my pen type thermometer and PID with PT100 into work and tested them with our thermowell and calibrated pt100. At room temperature, the pen was under by 3 degC and the PID was with 0.5 degC but at 100 degC the pen was within 1 degC and the PID was out by over 8 degC.
While this may seem alot, the 4 wire pt100 probes we use for reasearch in work cost a lot more than these normal homebrew sensors and the DAQ system cost the same as a family car and we are still easily calibrating by +- 2 degC on a regular basis.