Just throwing this question out there. Has Mueller's investigation veered out of control, crossed ethical lines and become a partisan witch hunt?
I do not think so and recent developments support my view.
First, it's worth noting that Muller is a Republican who, even now, is highly praised by certain high-ranking Republicans. While other Republicans are critical, it's difficult to make a logical case against Muller on simple partisanship grounds.
Regarding ethical lines, we'd first need to know what ethical line or lines exactly you are talking about. "Ethical lines" is a broad term. A clear answer is not possible without a question that is itself more clear.
Regarding "witch hunt," that too is a broad term. Before agreeing or disagreeing that the Muller investigation is a witch hunt, we'd need to know exactly what you mean by the term. I looked up the term in Wikipedia and a couple of dictionaries. If those definitions are used, I would say the carefully-conducted, Justice Department-supervised, and court-influenced investigation is not a witch hunt. While Trump loves the term, labeling Muller's investigation a witch hunt does not make it so.
Regarding control, Muller's office did not obtain the search warrant in which a federal magistrate permitted the FBI raid of Cohen's home, office and hotel room. It seems Muller's team came across information that gave them reason to believe certain crimes have been committed, but investigating those was outside Muller's mandate. Accordingly, they referred the matter to the New York Southeastern District. That office, (led by a Trump appointee who Trump personally interviewed before appointing him, and who recused himself from this matter) obtained the search warrant and conducted the raid.
In this example, Muller exercised restraint to stay within his mandate as he perceives it. Eventually, we'll all be in a better position to answer your question. But for that to happen, the public must know what was investigated and why, and how the decisions were made to follow the investigative tracks that were followed. I expect that info to become available when Muller issues his report and it finds its way into public view. Until then, we can't properly answer your question because we don't know what the Muller team is doing. We get glimpses from time to time, but by its very nature, the full story cannot be known until the report is issued.
In the meantime, my opinion of what is developing is strongly influenced by Trump's behavior and comments. His words and actions are not those an objective observer would expect from an innocent man.
Finally, it's important to note that the NY SE District office is not investigating Trump. It raided Cohen's office, home and hotel room, not Trump Tower. The Justice Department has had it's eye on Cohen for a long time. For more about that, see this:
A Brief History of Michael Cohen's Criminal Ties
Depending on what is developed by those investigating Cohen, Trump may be implicated, but we're not there yet.