I think the crux of whether this process is fair will be whether Schiff and Nadler will allow the Republicans to call Hunter Biden as a witness. We all know the answer to that, yet it is crucial to determine whether the President's interest in Ukraine investigating Burisma was for narrow political purposes or for reasons of interest to the United States.
Another thing that needs to be pinned down is what the quid was for the President's quo. I don't believe the infamous phone call addressed the matter of military aid at all. Instead, Trump and Zelensky talked about the possibility of a face to face meeting - that is, a photo op that would benefit Zelensky politically. It is one thing to ask Zelensky to investigate the Bidens and Crowdstrike as a condition of receiving military aid appropriated by Congress, and quite another to establish that condition in exchange for a photo op.
Of course, as we all know, the military aid was received (in a complete reversal of Obama's policy of only providing Ukraine with blankets and MREs), the photo op with Zelensky took place, the Ukrainians never investigated the Bidens, and the text of the phone call was released for all to read. The Dems are willfully acting as blind men describing the elephant. Take off the blinders, and look at the whole elephant - no crime was committed, and no cover-up has taken place.