Is That Flame Sword Stunt Really That Risky?
After watching the interview of Michiko Makino, she had revealed that the fire sword stunt had a prop sword wrapped in oil-soaked rags and placed on fire. It was really that hot! As much as some may call it the "flames of courage" but at times, I felt it was too reckless and not to mention too risky to be given more screen time.
In Kakuranger, there was the time when Sasuke used the Flaming Shogun Sword in episode 20 against the Hanarangers. If one notices the scene, the flaming sword wasn't used all the time. I guess they used more than one prop sword filming that awesome scene because it's a very risky scene to do. As for Muteki Daishogun's finisher, it was the usual use of already filmed stock footage for robots executing their mecha. Goseiger gave the same treatment for its robot Gosei Great's finishing move.
In Shinkenger, I guess they wanted more screen time for the flame sword so the only safe answer was to use CGI-generated flames. Notice that the actual burning swords don't get so much screen time while in Shinkenger, the flame sword has been given more screen time than actual flaming swords. If they would use an actual flaming sword, it would be too risky to let the stuntman do more screen time with an actual flaming sword than if CGI was used. Also, using real flames on the Rekka Daizanto (which was a very huge sword) would be too dangerous for actual use. Though a compromise might have been done like film a scene with both real flames and CGI flames.