No, it's hardly about "balance" for achievements. To begin with, Lucky Nations long predates the implementation of achievements.
Right. But is anyone here seriously saying that having the option for lucky nations is a bad thing? I thought the only serious arguments against it were caused by the fact that ironman, the achievement getting mode, locks it in. So yeah, saying that it's there for balance for achievements seems perfectly reasonable to me.
Lucky Nations does little to nothing to stop the player from defeating a lucky AI nation. It just means that in contests between the AI, the lucky nations will almost always win. The AI's ability to plan and fight wars is so poor that lucky nations is basically irrelevant.
Anyway, I don't play EU4, just EU3 (I mean, I have played EU4, but then I went back to EU3). So maybe something has change that I did not notice? In EU3 saying that lucky nations always win is totally untrue. It's not even that huge of a advantage. Now, perhaps something in the balanced has changed between 3 and 4, but looking it up the bonuses from being lucky don't really seem to be that much more. And I sorta am doubting the game is different enough for it to actually matter THAT much.
Also it seems a little disingenuous to say that the lucky bonus is huge enough that in a AI vs AI fight the lucky AI almost always win, and in the next sentence and paragraph go on about how the lucky bonus means so little. I'm not sure what you even think about lucky nations, whether they are too strong or too weak. (Although I would guess by the fact that you seem to dislike it having a effect on your game you think it is too strong?)