Crotalus told me there is a problem with this ability the SCU has. The ability is this: when something is destroyed in your base and leaves wrecks, a patrolling SCU will rebuild it (it's automatic, so it's not always a good thing).
However, it makes SCU bad at assisting and building, they don't use their range, they wait to assist that the builder has already started to work, and ground assist doesn't work. Removing Rebuilder ability makes SCU better engineer. But it removes this feature. So should it be removed because it's better to have good SCU engineer than rebuilder? OR is this ability very useful for certain players?