There's been a fair bit of commentary regarding House Judiciary Chairman's Jerrold Nadler's subpoena for the full, unredacted Mueller Report. Nevertheless it may be helpful to go over the issue a bit more.
To begin with, the Special Counsel as we currently know it follows from provisions in the Ethics in Government Act. Those provisions were allowed to expire in 1999, but were replaced by the current DoJ Special Counsel regulations (28 CFR 600). While the previous Independent Counsel provision under the Ethics Government Act had been upheld by the Supreme Court (with only Justice Scalia dissenting), it had come under fire by both Democrats and Republicans
Justice Antonin Scalia provided critiques of the Act, based on both Constitutional law and the potential for harm in practice, in his dissenting opinion in the case Morrison v. Olson. Justice Scalia, a judicial conservative, noted that the U.S. Constitution granted consolidated power to enforce the law exclusively to the Executive Branch. The Act extended the power to initiate criminal investigation to the United States House of Representatives and the Senate, which Scalia viewed as a violation of the separation of powers. He believed that the House of Representatives' investigation through the use of a special prosecutor "[arose] out of a bitter power dispute between the President and the Legislative Branch".
Even some Congressmen who had voted for the Act criticized it privately, leading to speculation that "if it had been an anonymous vote, it would have been voted down two-to-one". ...
Specific provisions of the Act have faced criticism as either too weak or too strong. The Special Prosecutor created by Title VI was empowered to pursue investigations with minimal evidence, and was required to pursue any accusation that the District Attorney could not disprove. It allowed for legal harassment of political opponents, even in cases that prosecutors stated they would have dropped in any other federal court. ...