The latter type is more common and significantly cheaper as less gold is needed to cover only the solder pads.
While more expensive and require more processing steps, they have several advantages, including excellent surface planarity (important for ball grid array component mounting), good oxidation resistance, prevents 'copper migration', and suitability for movable contacts such as membrane switches and plug-in connectors.
Early ENIG processes had poor adhesion to copper and lower solderability than HASL.
In addition, a non-conductive layer containing nickel and phosphorus, known as "black pad", could form over the coating due to sulfur-containing compounds from the solder mask leaching into the plating bath.
[1] The quality and other aspects of ENIG coatings for PCBs are covered by IPC Standard 4552A,[2] while IPC standard 7095D, about ball array connectors, covers some ENIG problems and their remediation.