Hanzou
Grandmaster
That variability is true of most combo drills. Even a jab-straight combo, their reaction can make that not a good choice to continue. Yet we (everyone in every art I've seen training for) trains combos. Done well, it acknowledges the variables exist. Ideally, the combos you work the most in drills are ones that have a high payoff (definitive chance at a win, or are often available).
Well the combos you use the most are often the safer ones as well. The jab is a very safe combo starter that you can work multiple punches off of.