Well all I can say is I've never had a flat file that failed to fully harden. I quench in Parks50 abd have my heat treat numbers down, so maybe that helps. As for horse rasps, I've had a few Nicholsons fail to harden but then I've had some Belotas harden and make great knives.
I make file knives because I think they are cool. Kinda neat to make something out of grandpa's old tool from the 30's. My hunting knife when I elk hunt is a Nicholson file. Skinned many an elk with that so I know it's good steel.