Even doing comp itself is sometimes vague because like I say, comps high and comps low has three to four standard deviation, in a one million home, the range could be from $850k to $1.3mil.and to make even thing more complicated even the same house that was sold for X price in March, same house could go to 0.98x by December due to seasonality aspect of neighborhood and supply-demand dynamics.From statistical perspective, zEstimate is good when there're lot of similar comps in the neighborhood with condensed price pattern (for example, if neighborhood price is ranging from 200k-250k ; but it would be all over the place if home price is too low or too high).Even professional desktop appraisl would face similar problem as well, so I would not blame Zillow/zEstimate. this is merely statistical issue.So you still need professional appraisal.But again if you ask three different appraisar, they would come with three different opinions LOL , it's the reality.

Check the leases.Are they weak and did the landlord give concessions (free rent,reduced rent,low to no deposit to move them in).Do you see a pattern for the area of reducing rents??

Moving Migration Patterns Report | North American Van Lines

The market also appears to be exhibiting its normal seasonal pattern with slower sales and higher inventory in the fall/winter months.Here are the full stats for Austin and the greater metro:Despite the recent macroeconomic headwinds, the Austin metro still boasts one of the strongest regional economies and job markets in the US.

It seems to be a pattern. 

For rentals I'd rather look at payment patterns of accounts myself.How soon will you take ACH type payments?

That portion of the fact pattern I am clear on as I've built and sold condos permitted this way in the past.

If you dont want to go on this pattern, then i would do 2-3 quotes per job.

I would also request their crime stats and break-in occurances of all properties in their care for the last couple years, at least, to check for patterns.  

You'll start to notice patterns to determine what is a "good" rent roll vs a bad.