Unless you are a medical professional, how are you able to determine what is necessary or unnecessary to reach a medical decision regarding certification?
The FAA uses designated authority with medical examiners, aircraft inspectors, pilot and mechanic examiners, etc. Where the FAA does not enable designated authority to make a final decision (AME's do not have discretion or blanket authority to issue a medical certificate for all conditions), the Administrator has reserved final decision for the aeromedical branch to make a special issuance.
By its very nature, special issuance means an exception to the standard: medical personnel (doctors) determine, based on requested tests and data, that the applicant is able to perform safely for the anticipated duration of the medical certificate, with a particular condition. Obviously this is subjective to the judgement of the medical professionals involved, as nearly all medicine is.
There are AME's who are very familiar with the special issuance process and who specialize in helping airmen through it, and there are others who will defer to Oklahoma City if there is any question at all. Most medical examiners sit somewhere in between.