This is the real problem.
Even if they have the historical data for that exact house/unit, it won't help them in cases such as:
* That nice view of the woods out the window is now blocked by a massive radio antenna that was just built there
* The river running through the back yard is now heavily polluted by something up-stream
* The new neighbor across the street is a huge nuisance and says they will never move
* The house just had a mass-murder event in it
Just because something is now cheaper than "comps" at price/sq ft and other metrics doesn't mean it's comparable.