Answer is “it depends” I’m afraid. They can measure signal delays on larger nerves, but have no real way to measure the smaller peripheral ones - that is done through biopsy (and not always 100% conclusive either).

In my case the results were 100% conclusive and enough for me to be determined “disabled” without any further questions.

Needles sting a bit but can’t remember any real pain from them.

Disclaimer: my info/experience is 6 years old, so there may have been improvements I’m not aware of.