"Balanced" is being used subjectively here. The game is not (or at least is not intended to be) weighted in favour of any great power, so it is quite balanced in that respect. This is appropriate for multiplayer I should think.
It is not, however, balanced towards certain powers gaining their actual historic extent, which I believe is what SonofWinter and others are referring to.
So really before you can say if the game is balanced, you need to
define what
you personally see as balanced.
From a modder's (or at least my own) perspective, the game is wonderful, as Paradox has provided us with a superb
tabula rasa ('clean slate', sorry I couldn't resist the Latin

) on which to build whatever mod we wish.
So overall, answers to the initial post need to be qualified as to what each person was looking for when they bought the game.