Since I started using the Libre all my sensors have averaged a little over 1mmol/l too low.
I don't blame the manufacturer, the sensors or the readers - because the result is so consistent, across 20+ sensors, and other people report similar.
Instead I 'blame' my body and my reaction to the sensor. The difference between finger prick and sensor reading is most inaccurate during the first three days of use, becomes more accurate for the next 7-8 days, and then loses accuracy a little in the last day or two.
I appreciate that this is much more problematic for insulin users than it is for myself, since I do not use insulin.
However, for me, the benefits of the sensor far outweigh the question of a fairly consistent inaccuracy.
There is a gadget called the miaomiao which can be bought separately, worn over the Libre sensor, that allows calibration of the readings.