The folks building Northshore in Canada have one huge advantage, which is access to a pretty much an unlimited supply of fallen cedar which doesn't decompose terribly quickly.
The manky pine that many pallets here are constructed from is thin timber and it's also pretty low grade. In the outdoors environment it doesn't last terribly long.
Some pallets are made of longer lasting hardwood so they may survive better but the planks are still pretty light weight if you plan on constructing any kind of long term structure.
Please, if you're going to build timber structures, do it either on your own property or somewhere where the land manager has given permission. Half arsed construction standards and illegal building make MTB'ers look like an environmental problem rather than a legitimate user group and make gaining access to areas to ride even more difficult than it presently is.