This is a relative question ...

This is a relative question as open wells could generate as good a yield as a borehole, depending on the aquifer transmissivities. The Ranny wells - open wells with horizontal galleries could generate great yields if installed in alluvial gravel in a yielding aquifer. Certainly, wells could become less hygienic if not well protected, so for relative water quality, you expect a borehole to generate a better quality water.