Is "unstructural plywood" suitable for shelving carrying a microwave oven?
From my reading, unstructural is not ok in damp/wet or areas with temperature changes. Microwaves put out a decent amount of heat and moisture, so I would not use it in this application.

Personally, I like to use solid wood for shelving as it is pretty cheap (pine) and you can normally get it in a size/length very close to what you need.