With improving technology, Continuous Glucose Monitoring (CGM) is as accurate (or as inaccurate) as most of the currently available glucometers. The exact accuracy would depend on the device and technology that you are using.
Most available CGM and glucometers have inaccuracy ranging from 10-15%. However, CGM devices are not used for accuracy, but they are used for trends. If accuracy is what you desire, then the only ‘accurate’ system is a plasma glucose measurement from your laboratory. CGM is NOT a substitute to the lab glucose values but rather provides valuable insights into diabetes management.