Yeah, a strong serbian economy is obviously good, but the thing is that Serbians don't really have many ways to get a strong economy going in the first place.
True, while Serbs can struggle a bit with their eco, they are not really handicapped in any way. They have good age-up politicians and the card that makes aging up cheaper is pretty nice too.
I can reach Age 3 in quite short time and have spare resources to make a decent army for defense with that strategy.
Once I get to Age 3 all I need to do is ship the card that makes the aura bonus even better and it's 95% GG.
The only way to bring down Serbia is with an almost perfectly executed early rush before the 7-10 minute mark. The only 3 times I have been defeated while playing Serbia were because of this, and admittedly, it's quite hard to execute such rush. I was only beaten by one of the best players in WoL when playing Serbia, that says a lot.
By the way, I just played Peru and noted their after-death dmg aura bonus doesn't work the way it does for Serbia, so the only concerns would be the Habsburgs and Serbia itself.
I really suggest to put a limit to the increase of this bonus. Don't nerf it, just put a limit to the irrationally high amounts of dmg. Like the others below have said, just limit to a 200% increase or so would do the trick just fine.