This article presents some possible downsides. It also mentions that much of the research on body cameras improves relations between police and civilians.
http://www.govtech.com/em/safety/Body-Worn-Cameras-Negative-Effects.html
Tony Katz is a radio personality that imposes spin on the stories he talks about.
The Tony Katz story links to here :
https://www.newsweek.com/police-body-camera-incident-report-memory-civil-rights-minority-711584
Which in turn links to a report here :
https://www.upturn.org/reports/2017/the-illusion-of-accuracy/
The fear is :
Police officer remembers that a scuffle started with the citizen striking the police officer.
Without review of the footage :
Police officer writes report saying the citizen struck officer first.
With review of the footage :
Officer sees that the citizen in fact did not strike first, formulates new rationale for striking the civilian first.
Result : Loss of transparency and truth of the view of the officer in the incident.
Probability is high that this will disproportionately affect black persons and minorities.
Better understanding of the claim :
Police officers reviewing the footage before writing a report allows officers access to details that can let them spin a narrative from their memory and the details in the footage. The opportunity to be unfair exists when police officers can view footage before recording their account of what happened. Perhaps allowing the civilian to review the footage at the same time as the officer and before writing any statement from their view would help to level the advantage. But, then again, who trusts the civilian from the street as much as the police officer.