Mechanical Nightwalkers are zombie-robots based in the Steampunk engine, created by the Professor. Compared to normal nightwalkers, they are stronger, appear to have higher health and they will charge up with electricity (you can see it flowing through them) then lunge at the player with electricity in their hands. However, they only just have really strong armor that makes your weapons seem weak and makes it look like they have lots of health. They are also counted as demons so weapons with supernatural foe damage bonus will still have bonus damage to them. They will only appear during the Professor's mission line, and not in standard demon missions. When they die, you can see electricity flowing through them and they explode.
Melee weapons such as Scythe,Terror Talons,The Mutilator are preferable in fighting them, as those weapons don't drain ammo in killing them and you can kill them with very much less effort with these weapons.