one of the biggest problems with sci fi is they make the humans eat the food on the alien world that of course is a problem because we cant even eat most of the stuff here on earth even if the aliens were made from the same bio chemistry eating the food would be like walking into a toxology lab and grabbing th nearest bottles and swallowing the contents you would be dead. Another problem with sci fi is the idea of aliens being killed by the common cold. thats virtually impossible because viruses are specifically designed to make one type of cell make new viruses thats why we cant catch ilnesses from animals (usually) an aliens cell is going to be completely different.