One of the least-publicized strengths of FreeBSD is that an entire cluster of machines is dedicated to continually building the Ports Collection, for each of the major OS releases and for each Tier-1 architecture. You can find the results of these builds at package building logs and errors.
Individual ports are built unless they are specifically marked with IGNORE. Ports that are marked with BROKEN will still be attempted, to see if the underlying problem has been resolved. (This is done by passing TRYBROKEN to the port's Makefile.)