Bond issuers can default. Then they'll go the other way. Again and again and again. Investments outside the U.S. are subject to fluctuating currency values. Over time, currency fluctuations tend to balance out.
Diversification helps, too.
Unstable political and social conditions can hurt investments overseas. A Consumer's Measure of Risk. Houghton Mifflin Company, Boston, 1994. Volume XVII, No. 5, June 1995, p. 31.