Source: pp 108-109, Is Eating People Wrong? Great Legal Cases and How They Shaped the World (2011) by Allan C. Hutchinson
[...] For example, the difference between Plessy and Brown has nothing to do with their legal integrity as a matter of constitutional doctrine. It has everything to do with the changing currents and concerns in the contemporary political context. Plessy’s separate-but-equal dogma ceased to be a fixed point on the constitutional compass because it no longer enjoyed sufficient political confidence and public support, at least among the elite. When Justice Harlan in 1896 predicted in his dissent that the Plessy judgment “will, in time, prove to be quite as pernicious as the decision in Dred Scott,” he was talking about its substantive merits as a political outcome, not its formal qualities as a legal judgment. And he was correct.
I do not understand the distinction by the bolded: What are the differences between substantive merits as a political outcome vs formal qualities as a legal judgment? I do know of the distinction between law and politics; some judgments reflect judges' political opinions than real law.