IMO returns are less critical than flexibility after RE. Most in the FIRE community are looking at withdrawing 4% or less of their portfolio per year. This means most people will have ample notice if things are starting to look truly critical, whether because average returns are poor for a long period, or because of a major recession or depression. At that point, you're presented with a series of options. You could go back to work full time, you could find some sort of income part time, or you could reduce expenses.
If you've retired in the developed world, and you're dead set against going back to work in any way, you have a lot of options when it comes to retirement locations in the developing world where your money will go a long, long way further. If you've retired on a cut-to-the-bone budget in the developing world, you've got much more limited options.
TL;DR: People who are inflexible will have a much harder time weathering sub-optimal conditions in RE. People who are flexible with regards to lifestyle, but who failed to save sufficient margin will also have trouble.