Despite its scale and complexity, only a small number of connection investigations find a technical NBN line fault. There is usually a simple explanation.
Sometimes the user's NBN plan is inadequate, or the modem is faulty or inadequate for the property or customer usage.
More education would go a long way towards reducing customer dissatisfaction when there is a perceived fault. The whole industry must work together on this. The goal would be to better educate customers using easy-to-understand communications that simplify the technology and improve troubleshooting.