Most units these days have a condensate pan that uses condenser heat to evaporate condensate from defrosting.
Using a freezer as a refrig may upset the balance between typical condensate level and evaporating pan size. Refrig's enclosed area is generally exposed to greater amount of humid air exchange than a freezer due to more door openings and refrig front opening door allowing cool air to fall out lower area and be replaced via top area by warm humid air.
Freezer with top opening door allows less environment humid air intrusion when opened but as a refrig it will likely get opened more often than a typical freezer use case.