The target dummies can be deceptive as they just stand still, unlike actual players which will actively try to avoid your gunfire. With McCree you just need to hit them 2-3 times and you're done, but with Soldier you need to have the exact same aim as McCree, but also the ability to reliably track enemy movement at range.

In the case of heroes like Reaper and Roadhog things are pretty even, but when it comes down to shooting Genji, Tracer, or Ana things quickly takes a turn for the worst as their tiny heads are insanely hard to hit - even for professional players. So when you have these important but slim characters that are hard to hit, you're better off being able to practically kill them in one shot than hope you'll be able to spray them down with multiple bullets before finishing them off with a slow-moving rocket.

Once again, Soldier can still most certainly kill the same sort of targets as McCree, but the skill needed to do so is significantly higher. And this is without considering Flashbang, one of the best abilities in game when it comes to ensuring a kill on a slippery target.