Every comp appearing in final report will have been verified with call to either (and sometimes, both) agent name shown in MLS update after the sale. Since I am a "zealot", as well as an "apostle", that primary question which usually needs to be asked concerns concessions to the sale. Some agents do post comment of their concessionary amount in their listing, but not the majority. I do not have difficulty contacting agents, nor in receiving answers to questions posed. The worst that can occur is to hear that so-and-so (lister) is out of town for another few days......and so I will call the selling/buyer agent. Without doubt the most common and important information that is lacking is clear description of amount of finish (% of s.f.) in a basement area, rooms there, whether a bath on main level shown as "1" bath is actually just a 1/2-bath, and updating details for kitchen and baths depending on age of the home. Our Building Dept. (permit issuer) has good online records for info from ~mid-1980's for other key facts about.....original permits, new rooves, replaced furnaces and water heaters, basement finishes, additions, added detached garages, etc......when MLS will not likely offer historical/date perspective well enough.
It is comical to see an agent's comment for "newer" this-or-that appearing in their listing write-up, i.e. "newer roof".....and you learn that it was installed in 1992 ! I guess the real meaning of "newer" is something NOT being original to the house. Sometimes you have to ask for specific details when a home is described as being......."Updated, or upgraded, or remodeled, or renovated, re-finished or rehab'ed". Well, what exactly happened ? What was done there ? Those words are not inter-changable.
At top of grid column I do indicate which agent was contacted for verification purposes and I write notes in red ink upon my MLS print page for info they stated over the phone. You ask about .....cost/benefit factor. I can't say there is a "cost" other than the time spent to pursue that information. The "benefit" is not getting calls from client reps or reviewers because the report shows plenty of informational detail.....and generally answers any question if they just read the entire report. Being thorough, clearly written and reasonably concluded does not mean that the value opinion will automatically match every one else's expectations........and that is where one truly "loses" in the greater run over time. It takes a lot of courage for a client to continue to send assignments your way, if along that way, there are bumps in the road and described property conditions and supported value opinion are NOT what the property owner had told the lender !