To my knowledge, there is no evidence that Hunter in particular was ever under investigation. The company he sat on the board of was officially being investigated.
I don't know what you consider to be an "non biased" source. I have a feeling we would disagree on who those are.
Here is a link to a bloomberg article about it. I found several others that also discuss this if Bloomberg is "biased" in your opinion.
Burisma Holdings was founded in 2002 by a corrupt oligarch with ties to the corrupt government of Ukraine. Hunter Biden was named a paid board member of Burisma Holdings in April 2014, 2 months after the UK had starting investigating Zlochevsky (the founder).
Ukraine didn't really do anything to assist the British in their investigation of Zlochevsky.
Shokin became prosecutor general in February 2015. Shokin took no action to pursue cases against Zlochevsky throughout 2015 according to his deputy at the time, Kasko.
The U.S. stepped up its criticism in September 2015, when its ambassador to Ukraine, during a speech, accused officials working under Shokin of “subverting” the U.K. investigation.
Kasko resigned in February 2016, citing corruption and lawlessness in the prosecutor general’s office.
Officials in the US embassy in Kiev began pushing for Shokin to be removed and eventually Biden went to force the Ukrainian government to take more action and get rid of Shokin.
I have yet to see any evidence that Hunter was actually closely involved in the actions of Burisma Holdings. He was put on the pay roll around the same time as several other high profile or connected people, likely in an attempt to curry some favor. Some others were a former president of Poland and the Director of the American CIA’s Counterterrorist Center in the George W. Bush administration.
It doesn't look to me like Hunter ever actually did much for the company and therefore would have risk of being caught up in a corruption investigation. Therefore Biden would have no motivation to go after a prosecutor to protect his son.
This whole "scandal" is just an attempt to smear Biden. That being said, I still consider Hunter profiting off of his father being VP to be scummy. Although less so than Trump's directly profiting off of being president.