One issue (apparently a feature) that may arise is that, if application is rejected in consular proceeding, the applicant is locked out from usa. AFAIK, if someone applies for an immigration visa in usa, they will not be able to obtain non-immigration visas in the future. A refused green card application might be the end of being ever in usa. The person may have to truely exit USA since there may be no way back (close bank account, sell property and assets, etc).
If the person adjusts status in usa, there are more possibilities for appeal etc.
If the person adjusts status in usa, there are more possibilities for appeal etc.