The friendly team at Snap make the usually slow process of rental pick up surprisingly quick. They are good at what they do, from shuttle service to car hand over. In typical kiwi multi-task fashion, the shuttle driver loads/unloads/assists and then mans a check in counter, to keep things flowing. The Snap depot is close to the terminal (9 Manu Tapu Dr at time of writing this) compared to some similar companies, and now that the ‘in terminal’ hire car companies are no longer in the terminal building, but in an adjacent building across a series of paths and crossings, the shuttle inconvenience is less significant, though not nonexistent. You should of course allow a little extra time at vehicle return in case of unlucky shuttle timing, though my experience was perfect: park car, handover keys without queuing while my 2x bags were loaded, into the van and immediately away to drop point.
Only suggestion would be slightly clearer instructions about where to meet the shuttle, and any back up process such as what to do if no shuttle in sight. Better airport signage (specifically directing to off site car hire shuttles) would help here. Maybe a photo of the shuttle van on confirmation email so we know what to look for.