Engineers in Assaults

[Dog Soldier]

IIRC from the User Manual, "However, in an assault there are two combat results calculations that are performed simultaneously."
That said, I would refer to the assault values of engineers versus infantry and other units you might consider in an assault where you could add the engineers.  Typically, these assault values range from 20% to upwards of 50% increases over just regular infantry.
I also suspect, your careful use of engineers in assaults meant the defending units were not only disrupted in most all cases but also at least yellow fatigue.  While the attacker cannot see the fatigue value of the enemy units, it can be estimated by the fire attacks placed on the target hex enemy units prior to the assault.  This estimate is a  result of experience, so it it is not always correct.
These factors combined can certainly reduce the assaulting unit casualties.

A simple example I would suggest is playing the Soviet forces and assaulting an Axis held hex with only one Soviet infantry unit.  You might well take the hex, at a high cost to your assaulting infantry.  However, there are ways for playing the Soviets to mitigate such losses.  As always, it just depends on the situation.  Time pressure to take a key position or VP hex, and the player's appetite for risk.

Dog Soldier