Appraisals values are data influenced opinions of value, not Gospel Facts. Yes, appraisals can be challenged and overruled opinions happen all the time. In many cases what's needed to get a "true" (not "higher") value does not exist.
When challenging an appraisal a buyer must provide 2-3 additional comparable closed sales sold within 90, but no more than 120 days from contract date. The key words of "additional", "closed" and "date" are invariable. A buyer cannot re-litigate the comparables used or provide a 2nd/3rd appraisal with the same comparables used as these comparables are also "opinion based valuations" and not, as hoped, Gospel Fact either.
All appraisals are reviewed. This means that you have to not only convince the appraiser that their work was insufficient, but also the reviewer who concurred with the original value. It's a high bar to hurdle, but one that I've seen good data miners jump over before.
PS: The success rate is roughly 25 percent - clearly an uphill battle. Get 2-3 lenders working on the same deal to start with. Yes this means paying for 2-3 appraisals. Yes also, like playing 3 separate hands of Blackjack at the same table in Vegas, your odds of beating the Dealer are exponentially increased. Perhaps the home is overvalued. With 2-3 appraisals in hand a good listing Agent can convince a seller to reduce their price.
My .02c
