You are comparing apples to oranges. ENIG describes materials (nickel and gold) and processes (electroless for nickel and immersion for gold). Flash describes the plating thickness. Flash typically means a very thin (3 to 10 microinches) coating of gold, regardless of the process. Therefore, flash gold could be electrolytic, electroless, or immersion. Initial problems with flash gold in the 1990’s tend to refer to an electroless process used over separable connectors that resulted in galvanic-driven pore corrosion.