You are simply wrong on that, Sturmpioneers plant their mines at exactly the speed Pioneers plant a single S-Mine field.
On top of that, you are wrong about S-Minen as well, they do 40 damage each and (unlike all mines except Soviet tripflares) are capped at killing one model maximum. Four S-Minen thus have a damage potential of 160, which is just enough to kill two models. They also have less penetration than all other mines, allowing even a Universal Carrier to drive over them without taking any damage, and they come with fucking signs around them.
Ergo: If you lose a squad to S-Minen, you are either among the most stupid players of this game, or you didn't keep an eye on your backyard (to prevent a player laying mines in your retreat path) and didn't use healing, which means you are among the worst players of this game. Take your pick…
Incomplete mine fields can be instantly deployed, so keeping an eye on your backyard really doesn't matter. Those mines can be laid under fire, creating a significant delay to disarm them.