Fiduciaries and people in official positions who exercise discretion are held to certain standards.
Such people are required to avoid conflicts of interest. A conflict of interests is when their personal interests might influence the discharge of their duties. The conflict might be that they have a relationship with a party, or they had an experience that might color their view. No one should accept his wife’s father as the judge in his divorce. A detective should not investigate the murderer of his son in a subsequent criminal matter.
It is not an issue of examining their decisions for bias, it is a matter of not placing them in the position of having a bias. So all this talk of the FBI officials who clearly expressed a bias against Trump not proved to be influenced by their bias is a side issue. They should not be on this case if they are biased.
It is also a non-issue to say that they are entitled to political opinions. Of course they are, but some opinions are disqualifying for investigating a politician you despise. Assign them to investigate someone else, just as you would assign a judge to preside over a case without his daughter as a party.