The only reason I didn't like the Promethean Weapons was because they were designed to fight Flood, and Flood only. Yeah, sure there's an awesome disintegration effect, but they should have made weapons designed to kill other life forms too. Like weapons they had back in the Human-Forerunner War.
Well during the Human, Forerunner war weapons like Light rifles or Scattershot were used despite the Forerunners not knowing of the Flood.
It is quite possible the Forerunners reached the highest technology level they could achieve.
Although how these are the same damage as to bullets is.... interesting.
My guess is for balance reasons and in real Halo lore Chief would die with one shot from a Light rifle