It seems that this isn't desired. I personally consider that it would add more depth if some bonuses were stronger but they had downsides as well, but I don't know any national idea which is a two-edged sword. Imagine something like:
30% cavalry combat ability
+2% revolt risk
Aka - "careful, you're playing with fire over there!"
Actually I can't really name 1 stat in-game that has both a positive and a negative effect. What we have instead are opposing stats or stats that slide from negative to positive effects. The only one that I know of which could swing both ways, theoretically, is mercantilism. But in-game I think it only has a positive effect.
As far as national ideas are concerned I suppose I get why they don't want it - because you have absolutely no control over it: are you going to skip picking idea groups and filling them in order to avoid the downsides? But this point only applies to absolutely new players: anyone who has played more than 1 game will know to look at his nation's ideas before starting the game and then trying to pick idea groups to help him in his nation's context (including its national ideas)
