For me. it is that I have to depart my family at times.
A friend of mine lives in Germany. She is very sick. She has medical insurance in Germany but not China. However, she does not have family members or relatives there to take care of her. She had to stay in Germany for the treatment.
As for a lot of Chinese immigrants in foreign countries, there is a dilemma they have to face: how to take care of their parents when they get older and older? This question is even tougher if they are the only child. And due to China's long-time one-child policy, many of them ARE the only child. As most of their parents still reside in China.
This is not really a 'dilemma' but more of a challenge for me, when I travel internationally: 'Time differences'. I live near Los Angeles, California. The time difference between here and Beijing, for example, is 15 or 16 hours, depending on the time of year. When I was in Beijing two years ago, and in Qingdao last year, it was difficult to video conference consistently with family. I was in Beijing for 3 weeks and Qingdao for 5 weeks, so like you I missed my loved ones. Time zones more that geography created distance between us.