Absolutely no reason not to use the loads - with care of course. I've used Unique in my .45-70s for many many years, and BlueDot for quite awhile too. I now prefer BlueDot since it uses slightly larger charges and bulks up better in the case. Accuracy has been very good, recoil is low, report is not too loud either - all plusses for many uses.
Use caution if you try to increase the load data - pistol powder loads are not automatically low-pressure, and adding one or two extra grains of powder could put you into dangerous pressures. You do have a good margin of error with the Handi - chambered for the .500 S&W the action is very strong. Still, if you want higher power don't use pistol powders!