In my experience, targeting several countries (from the same tier) in the same Facebook campaign works well, although I had to accept that some countries will have a bad ROAS and I need to evaluate performance across the entire campaign. If I remove those "bad" countries the campaign enters into the learning phase again, which is not always good -- so it is hard to make those tweaks every time I launch a new campaign (add and remove countries).
When I try to target only a single country in a campaign to have a better control on CPI, it seems that Facebook has more difficulty in finding users and the CPI ends up being high.
Since bids can't be adjusted by country, what is the best strategy for Facebook campaigns?