None.
All of my Apple products, except for iPhones, have been refurbs.
The experience has been excellent.
I think they undergo a higher level of QC than retail units anyways.
Most of the reforms are probably just coming from people returning their apple products for ridiculous claims about screen brightness, color, artifacts, buzzing, too loud clicking, or any of the number of other mostly imaginary maladies that folks here wring their hands and get far too emotional over when new products are released. So let this be a big thank you to the folks that refuse to believe that they are "holding it wrong" 😉 .
The ONLY downside to the refurb store is lack of custom configurations. However, by the time I am ready to buy, it seems that I have a number of suitable configurations to pick from. If you require a special snowflake build (nothing wrong with that) then refurb store is not for you (nor would the local apple store).