I've worked with the NTSB on accident/incident investigations, and I've seen absolutely no evidence of bias or impropriety.
However, the FIRST thing we were told at the beginning of the investigations was that we were not allowed to publically release ANY information with regard to the investigation until the NTSB had issued their report - we were even discouraged to discuss it with co-workers other than to get additional technical input. Failure to abide by those instructions by an individual would result in severe discipline - including being fired. One of the most difficult parts for me when I was investigating Lauda was that I couldn't talk about it to anyone (it had quickly become obvious we'd collectively missed something that allowed the T/R to deploy and cause the crash, killing hundreds - and that was not something I easily internalized).
the NTSB wrote that both IPA and UPS took actions prejudicial to the investigation by publicly commenting on and providing their own analysis of the investigation prior to the NTSB's public meeting to determine the probable cause of the accident.
The NTSB did what they said they would do at the beginning of the investigation. I have zero problem with that. To have turned a blind eye to the misbehavior of the pilots unions would have been the NTSB showing bias.