One major downside of the wooden pallets is the ever-present risk of fire. I worked at a food/vegetable processing plant once, it'd get vegetables from the field, we'd pack them, chuck 'em in boxes and stack them on pallets. (vacation job, yay)
Next to the plant were huge stacks of wooden pallets. Years after I worked there, the whole plant burned down; a fire started in the pallet stack, and since they're awesome fire material they took the whole factory with them. Not sure if it was accidental or intentional though.
Bonfires and world record attempts for bonfires often involve large amounts of wooden pallets. Cheap, very open, stable/stackable, ideal fire material. There's a picture of the world bonfire record in Norway where they build a giant tower out of pallets.
Isn't fire also a danger with plastic pallets? Generally, plastic fire is even worse than wood fire (in terms of heat and toxic substances generated). Wood burns, but it isn't instantly flammable and may in some circumstances withstand fire better than steel (though this is counter-intuitive).
You can add chemicals to plastic to make it resist fire, but those have other problems, often with toxicity.
Generally, if you have huge stacks of some unused material in a place, and it can catch fire, there is something wrong with the process. It would make sense that those pallets are worth some money so you can get rid of them easily. If not, should the local authorities control the amount of material stored if it can be a hazard?
Next to the plant were huge stacks of wooden pallets. Years after I worked there, the whole plant burned down; a fire started in the pallet stack, and since they're awesome fire material they took the whole factory with them. Not sure if it was accidental or intentional though.
Bonfires and world record attempts for bonfires often involve large amounts of wooden pallets. Cheap, very open, stable/stackable, ideal fire material. There's a picture of the world bonfire record in Norway where they build a giant tower out of pallets.