So here we can see by IpKai's graph that the Sturmpioneers did in fact receive a devastating nerf to overall effectiveness, thereby making them useless.
When compared against other weapons they are still very powerful, they also had a reduced received accuracy modifier applied to them which increases their survivability. The modifier reduces damage from small arms sources by 0.87 which is quite significant.
A conscript squad for example at max range deals 4.38 dps however while shooting at a sturmpioneer squad they will only deal 3.81 dps.