Furthermore, donors take widely varying strategies toward liaising with government. For example, while World Bank funding largely goes through official government channels, the United States Government's President's Emergency Plan for AIDS Relief (PEPFAR) had largely bypassed recipient country budget-planning procedures in its effort to achieve a rapid scale-up of HIV/AIDS programs [14].
http://plosmedicine.org/article/info:doi/10.1371/journal.pmed.1001214#article1.body1.sec3.p3

Whether countries are diverse does not speak much to the choice of random vs. fixed effects. The question is whether the diversity is well-behaved in the sense of being normally distributed. Certainly, it may not be, which does argue for a fixed-effects model. But unless I'm misunderstanding, that's not the argument being made here.