"Brain drain" for countries? I'm not sure.
To be honest I'm not sure if I'm comfortable making any judgement as I sort of feel like I need to look in a dictionary to find out exactly how it's being used, because it's not 100% intuitive to me without an adequate context.
Frankly, I did not find any information in the dictionary to know whether it is countable or uncountable, nor did I understand how I can use it correctly!
My context was nothing more that what I said:
Many underdeveloped and developing countries have experienced brain drain; that is, a lot of people prefer to immigrate from underdeveloped countries to industrialized ones to thrive
But you didn't say if you'd tried to create that sentence yourself or if you found it somewhere, and if written by a native English speaker is a whole set of different responses trying to work out what they meant, rather than if we know you're trying to use it and we give our opinions on our judgements on it. Can you see how that's important information to give?
If two countries, call them unimaginatively Xia and Yland, each experience "brain drain," then there are two brain drains. They could be different in severity (% of college degree recipients leaving the country) or age (one might have started before the other). Now we can say that Xia is suffering a brain drain that is more severe than the brain drain from Yland, but that Yland has theolder brain drain (or an older brain drain than Xia). Xian Education Minister to Ylandese Education Minister: "My brain drain is bigger than your brain drain."
I am sure I have seen articles with the two-word phrase "brain drain."