The nature of any project is inherently fractal, and trying to assign a impact to each part is all over the map, and anyone with any agenda or bias can move the 1000 little sliders enough that it adds up to what they ultimately want to see.
You get stuff like:
"Lets assume all the trucks are old and need to drive up hill to deliver the panels"
"Lets assume that the solar panels are installed in a place where it never is cloudy"
"Lets assume the coal plant only burns coal from this one deposit on earth that has the lowest NOx emissions"
"Lets assume the solar panel factory never bother putting panels on their roof, and instead run on coal"
