New Yorkers manage to see storage opportunities in the most unlikely places -- an unused oven that can accommodate sweaters, for example, or a too-windy balcony that can be used for... What exactly it can be used for is the subject of a thread on StreetEasy.com: "We have a regular balcony space and NOT a roof or terrace space. We live in a new condo by the East River and it can get very windy at times. Fortunately, one end of the balcony is protected from the wind by the building so this is an ideal place to place the storage units."
The response so far has been cautionary, beginning with the need to gauge the condition of the balcony and how much weight it will support, particularly if it's old.
The height of the storage units is another concern. Children (not to mention dogs and cats) might be tempted to climb on top of storage benches, which might put them far beyond the reach of safety railing. The look of the storage may rile up your neighbors if the units are not neat, tidy and minimally attractive in a way that's in keeping with the building, and co-op boards in particular may not approve.