The first is scope. While Singapore and New Zealand are vulnerable to economic disruptions, both have different economic resources and areas of concern. These differences are clearly apparent in AOTES which basically trades energy for food.
But not every economic disruption will affect these two sectors in the same way, as the Iran war and disruption in the Strait of Hormuz have. Hence, the basic bargain may be less relevant if, for instance, trade is disrupted by a health crisis or conflict in another part of the global economy.
The second reason for some scepticism is scale. The benefits of such an arrangement could be harder to maintain with more partners.
