Ok, my first point is actually that it is to less random because when I have a mission one of the first USS mess with it (there should be an option to reject it)
All other USS seem to be random. They randomly pop up and they have a random event. Maybe it would make more sens if there where some hints (e.g. distress signal, scrambled transmission, trade offer) so you haven't to stop by every USS to find what you need. Also they should stay where they are or they should be global. If you miss your mission USS you have to fly back and not just wait till it pop up in front again. Maybe you can ask other ships if they have seen some (special) USS, so you have to search and not to wait.
All other USS seem to be random. They randomly pop up and they have a random event. Maybe it would make more sens if there where some hints (e.g. distress signal, scrambled transmission, trade offer) so you haven't to stop by every USS to find what you need. Also they should stay where they are or they should be global. If you miss your mission USS you have to fly back and not just wait till it pop up in front again. Maybe you can ask other ships if they have seen some (special) USS, so you have to search and not to wait.