I have a spreadsheet somewhere, I was tracking Soldato, had 56 games tracked if I remember correctly. Basically, it broke down to this. With one soldato, 70-75% chance the bomb doesn't go off. Two soldatos, slightly higher chance both bombs don't go off, and a slightly reduced chance 60-70% that the only one bomb doesn't goes off. Remember I'm looking at this as failed attempts.
So much can happen, as it turns out, to ruin a good bombing. All of these percentages go up when one or both soldatos do not plant the bomb N1. And the amount of times a bomb was planted but not detonated would probably surprise us. That is what I would improve @Frinckles . Soldato needs a tweak so people plant their bombs N1, OR mafia needs a way to be able to detonate bomb in the event of the soldato's demise, OR code conditions in which a bomb is triggered and will explode without the soldato detonating, because remember if the target dies before detonation, the role is worthless, and that happens a lot too. Those three things, any one or all, are what I noticed regularly stop soldato contributing to the mafia team.