I'm a firearms instructor with some law enforcement experience and keep track of what the typical duty weapons are for major LEAs (law enforcement agencies).
I don't believe the FBI has a grandfather clause for approved firearms. Once the FBI changes approved firearms, agents are required to turn in their service sidearm for the new approved service sidearm. The fact that Gideon, Reid and Rossi are allowed to carry something other than a Glock in .40 S&W is a technical error; a major one to a firearm savvy person such as myself.
Revolvers have not been approved for use by FBI agents since the Miami FBI Shootout of 1986.
On the alleged competition shooter who stated revolvers are reliable. Wrong. All you have to do is to bend the crane or extractor rod and the revolver will not function and will require a gunsmith to repair. If the cylinder ceases to rotate on a revolver for any reason, it will no longer function. A tactic to disarm someone with a revolver before the hammer is cocked and the cylinder has rotated is to grab the cylinder. It does not take much grip strength to disable someone with a revolver by grabbing the cylinder before the cylinder has rotated to fire the next chambered round in the cylinder.
The advantage the revolver has over semi-auto pistol is that an experienced shooter such as Jerry Miculek can empty a revolver cylinder faster than someone can empty the magazine of a semi-auto pistol.