Now that's a huge understatement. What the Slaughterhouse cases did was to essentially render the Privileges or Immunities clause null and void, which of course ultimately led to what is now the hopelessly convoluted doctrine of Selective Incorporation via Substantive Due Process.
Something you might enjoy reading is Justice Thomas' 1999 dissent in Saenz v. Roe. Snippet from footnotes:
Legal scholars agree on little beyond the conclusion that the Clause does not mean what the Court said it meant in [the Slaughterhouse cases]. See, e.g., Harrison, Reconstructing the Privileges or Immunities Clause, 101 Yale L. J. 1385, 1418 (1992) (Clause is an antidiscrimination provision); D. Currie, The Constitution in the Supreme Court 341— 351 (1985) (same); 2 W. Crosskey, Politics and the Constitution in the History of the United States 1089—1095 (1953) (Clause incorporates first eight Amendments of the Bill of Rights); M. Curtis, No State Shall Abridge 100 (1986) (Clause protects the rights included in the Bill of Rights as well as other fundamental rights); B. Siegan, Supreme Court’s Constitution 46—71 (1987) (Clause guarantees Lockean conception of natural rights); Ackerman, Constitutional Politics/Constitutional Law, 99 Yale L. J. 453, 521— 536 (1989) (same); J. Ely, Democracy and Distrust 28 (1980) (Clause “was a delegation to future constitutional decision-makers to protect certain rights that the document neither lists … or in any specific way gives directions for finding”); R. Berger, Government by Judiciary 30 (2d ed. 1997) (Clause forbids race discrimination with respect to rights listed in the Civil Rights Act of 1866); R. Bork, The Tempting of America 166 (1990) (Clause is inscrutable and should be treated as if it had been obliterated by an ink blot).