Re Bismark and France, the prussian army at the time was field proven and second to none, as Bismark well knew. The prussians had carefully prepared for the war and were starting to gain in the war technology tree, as they proved in their war with the Austrians, where they smashed a vastly superior army. The french completely failed to see the war coming and their post napoleonic army, unprepared and unproven, was great in name only. They were probably still mobilizing when it was all over.
I don't think Bismark had too many nightmares over it. He needed the war to achieve his ulterior motive, the unification of germany, and was willing to roll with the risks involved. In fact, Bismark went out of his way to make it all happen in the first place, it was the culmination of years of planning. A truly remarkable man.
WWI is a completely different kettle of fish. That was never meant to happen, it was not the result of a carefully prepared political plan, with all the parts in place. Once it happened, a carefully prepared plan was followed that was military in nature only. The political aspects were missing, as they were not with Bismark. When the von Schlieffen plan was conceived, there was no trans Siberian railine, and there was no entante codiale between Great Britain and France. The german military had little in the way of options other than to follow their dated plan, and they almost got away with it. Actually, they believed they would get away with it, just as Bismark did. In this sense, WWI disproves your case.
The hindsight argument is out of place here, as we know, within reason, what the people of the time were thinking, from contemporary documents.
Bismark couldn't know he would win in the sense that nobody knows the future...But it was a lot more than mere hindsight that led him to the gamble.