Clinton's impeachment stemmed from Ken Starr doing the same thing. Starr was appointed to investigate Whitewater and "anything" related to the investigation. Starr used this power to fold not only investigations regarding the White House Travel office, the White House (possibly in Hillary's office) accessing personal FBI files of people and civil cases against Bill Clinton for sexual harassment into his investigation and then Clinton lied to Starr about not having sex with Lewinsky. In the end nobody even cared about Whitewater, the Travel Office, FBI files or anything else, not even Starr because he had got an easy "win" by catching Clinton in a crime against his investigation. But Starr also couldn't exonerate anyone for the previous things because it's not his job to prove they didn't do it.
Mueller's easy "wins" were against people like Flynn and Manafort lying to his investigation. Also the Russians who never left Russia and didn't even bother to cover up their actions because they probably never plan on leaving Russia. (I mention this because Russia makes a point of not extraditing its citizens under almost any circumstance possible. No one charges them with crimes actually expecting to prosecute them normally.)