The ethnobotanical data analysis using the relative frequency of citation (RFC) to determine the well-known and most useful species in the area is much better.

You can use both depending on your research objectives and size of data obtained.

You can you both , depends on your study objectives . Also you can apply the use value and RVA techniques to analyse the conservation status of ethnobotanically important plants

You can use both, but usually we use RFC in quantitative ethnobotanical studies.