What is the accuracy of this methodology?

When input data quality is acceptable and after appropriate corrections are made, confidence in Mechanical Specific Energy (MSE) output is high.
Careful study of drilling reports and other data allows the analyst to detect, flag and attempt to correct for changes in drilling efficiency so that MSE is a reliable indicator of rock strength.
As with most data analysis techniques in the E&P industry, some level of uncertainty in the absolute value final output remains.
At a minimum, over a limited interval such as a frac stage, the output provides a reliable relative indicator of the lithological heterogeneity along the wellbore that can be used for optimizing frac design.
Using actual rock data in an engineered completion process requires more attention to perforating depth control, which is a potential source of “error.”