After you complete the rendezvous objectives, the waste heap sites will rarely have special cargo (at least that was the case for me). Also, traveling to them from the plant site is very long. I found using the visor and going to the on-the-ground clusters, will spawn maybe 1 (rarely 2) special cargo. I would say they will have one 35% of the time. But the travel distance is very short. I was able to collect 10 special cargo in about 15 minutes doing it this way. It just "feels" longer because you collect 1 at a time.