One thing which makes country-to-country comparisons "not so useful", is the huge amount of migration. Eastern Europe is "emptied" of carpenters, painters etc. because so many have migrated to Western Europe where the money is, and in some Western European countries Polish JWs make up 10 % or so of the total number. And then you have the opposite migration - if you see pictures taken from the witnessing work in some South Amercan amd African or Asian countries, you will se a large number of European or USA witnesses among them.
So countries are not that interesting to compare as earlier; foreign language congregations and groups should be considered in addition. And there are other factors such as death rate etc. A complex field, really.