1m² of crop requires 1m² of sunlight. If you have 1000m² of crops and 100m² of sunlight hitting the roof you will need to get that missing 900m² from somewhere else.
Depends if it offsets the inefficiency of watering. As water becomes more and more of an issue the power needed to run LED grow lights isn't the biggest issue.
I believe farms like this use a lot less water to grow the same yield.