Better question is "how bad are the compared to those of professional appraisers who look at half a dozen comprables?" An even more important question is "how biased (based on who is paying) are those professional appraisers?"
Actually computing the market value as a "cost-to-build-today" is rarely done, at least for home sales the typical consumer is going to be involved in.
And those appraisers can be very biased in either direction.
But lenders generally don't care so much about the cost to build, so an appraisal ordered by a lender generally won't.
That being said, my recent appraisal (I was financing the purchase) said "All 3 approaches to value considered with most weight placed on sales comparison approach."
I could mathematically prove this statement false, as the cost to build had zero consideration on the value. I think it is mostly a canned statement. Maybe they have a different definition of the words "considered", and "most". :)
[I]n 2007the New York State attorney general sued First American: relying on internal company documents, the complaint alleged the corporation improperly let Washington Mutual’s loan production staff ‘’hand-pick appraisers who bring in appraisal values high enough to permit WaMu’s loans to close, and improperly permit WaMu to pressure …. appraisers to change appraisal values that are too low to permit loans to close” [FCIC 2011: 92].