You should try to build your model as best you can, but you've got to be methodical in stress testing the accuracy of the model in all sorts of cash distribution levels designed to trigger and not trigger each step of the waterfall to ensure that your model is accurate.

This data is typically input directly by the listing agent or an assistant, which leaves some question as to accuracy on occasion.In any event, when performing a comp, you simply select properties that closely match the subject in terms of size, neighborhood, amenities, land area, etc. based on the fields available on that MLS.

It is not necessary for one or more of the comparable sales to be of the same design and appeal as the property that is being appraised, although appraisal accuracy is enhanced by using comparable sales that are the most similar to the subject property.

Without knowing how they're getting their data I have not idea as to its accuracy.

Trying to find a magic one size fits all formula or shortcut, and using it, is why many investors fail, and are then forced to overestimate on the side of caution, which rejects good deals, and screws up bad ones, still  misses the mark, and then they will criticize or call out those that are actually accurate...because accuracy can't be done.  

You cannot predict appreciation, but based on my experience over the last ten years, I can predict cash flow at a much higher accuracy rate.You're also right, cash flow can go down, but in Austin as well as the other markets I'm in that are experiencing tremendous job and population growth...hence demand for rentals...

Not a lot of people may know this but CoStar also owns Apartments.com, so there is some accuracy to the rent comps, vacancy, market info, etc that the database provides.

Crunching metadata can certainly increase the accuracy of your proforma, but there is inevitably events that will change the trajectory of the data.

For those who say "nobody can know what's to come", well, then explain how I did with near pin-point accuracy. 

With stock trading, you can monitor your results every minute ... even every second ... with great accuracy and make adjustments.