Well, do try to remember how powerful he made a small child with little to no training at all. Someone with that sort of knowledge is likely to have the skill neccesary to train a real fighter. As a matter of fact, I'm thinking that Happi's method was the hell he put them through. Look at what he's done to Ranma - The ultimate weakness moxi-whatever caused him to learn an entirely new technique, one that is considerably powerful.
Although you can look at that and say "well obviously he was just being a dick", it still caused Ranma to grow more powerful as a martial artist. His "method" of training seems to be causing problems for his "students" and letting his "students" figure them out, really. The problems he caused for Genma, i.e. teaching him to be a thief, in turn made Genma develop the ultimate thieving technique. Soun's Demon Head technique, although shown as not having any physical effects, is psychologically powerful, and could probably deter most pursuers.
And slightly off-topic, since Soun's real "power level" isn't shown at all in the series, one can not truly ascertain how powerful he was in his prime, or even how strong he is now. I know that there is something to evidence his strength, but I can't remember it at this exact moment and I encourage everyone to remember how in-consistent canon was in that regard.