At the Washington Post, former federal prosecutor Andrew C. McCarthy provides a balanced summary:

As he explores possible Trump campaign collusion in Russia’s election interference, is special counsel Robert S. Mueller III running an impartial investigation? …

In Mueller’s case, there are various grounds for worry. Mueller’s investigation was triggered when former FBI director James B. Comey, no fan of the president who dismissed him, leaked a memo of a meeting with President Trump. Comey admitted hoping this revelation would lead to appointment of a special counsel.

Furthermore, the investigative team Mueller has assembled includes Democratic donors and supporters, including one lawyer who represented the Clinton Foundation and one who represented a subject in the Hillary Clinton email investigation. This month, moreover, it came to light that two members of the team, who had also worked on the Clinton email investigation, were having an extramarital affair and exchanged text messages expressing partisan political views — favoring Clinton and depicting Trump as “loathsome.”

Worse, in one August 2016 text, one of them, FBI agent Peter Strzok, asserted that the FBI “can’t take that risk” that Trump could be elected, equating some unspecified action against this seemingly unlikely possibility to “an insurance policy in the unlikely event you die before you’re 40.” Dismayingly, this text, which crosses the line between political banter and tainted law enforcement, refers to a meeting in the office of FBI Deputy Director Andrew McCabe, then (and now) the bureau’s No. 2 official. While not as weighty, legitimate questions have been raised about McCabe’s own objectivity, his wife’s state Senate campaign having been lavishly funded by groups tied to Virginia Gov. Terry McAuliffe (D), a Clinton insider.

On the positive side, Mueller immediately removed Strzok from the case after the texts came to light. (The other texter, FBI lawyer Lisa Page, had already left the team.) Further, a critical metric of Mueller’s tenure is how he has performed. To date, he has brought three sets of charges: an indictment against former Trump campaign chairman Paul Manafort and his associate Rick Gates and two false-statements cases, against Michael Flynn (fleetingly Trump’s national security adviser) and George Papadopoulos (a minor Trump campaign adviser). None of these cases bears on the 2016 election or suggests any wrongdoing on Trump’s part.

Thus, while there is cause for concern, the results Mueller has produced so far appear free of political taint. And at least in the matter of Strzok, Mueller was scrupulous about removing what was, at the least, the appearance of impropriety. …

Personally, I am not much alarmed that several of Mueller’s staffers have anti-Trump political views. But as more evidence emerges, I have become increasingly disturbed about whether those views will taint perception of the Mueller investigation, particularly in the case of Andrew Weissmann, a key Mueller deputy. A gifted career Justice Department lawyer, Weissmann sent former acting attorney general Sally Yates an effusive email shortly after Yates was fired for insubordinately defying Trump on enforcement of the so-called travel ban. The obstruction aspect of Mueller’s investigation calls for an objective evaluation of how much independence law-enforcement officials have from the chief executive. Weissmann’s lauding of Yates suggests he is not objective on this point. …

I began with the belief that Mueller was a superb choice whose well-earned reputation for personal integrity would be critical. I still think so, but I’ve been shaken by his puzzling insensitivity to the imperative that his staff be, and be seen as, driven by evidence, not anti-Trump bias.

Just as Mueller would not have recruited Strzok had he known of the texts, one imagines he would have passed on Weissmann had he known of his email paean to Yates. Removing Weissmann, just as Mueller removed Strzok, would be a reassuring course correction.