I dont see a downside.
A special prosecutor would indicate those being investigated had insider power and influence to derail the investigation and consequences.
Standard Dept. of Justice/FBI procedures would indicate less influential subjects.
Normally special prosecutors are not requested.
Downside is damage of credibility of the Justice System.
We don’t have a “Justice System”.
We have a broken, corrupted “Legal System”.
The downside is only honest AGs will appoint one. Criminals like Holder and Lynch never will.