Absent the occasional flights of fancy events, most of which would not individually seriously unbalance the game (albeit because they represent ahistoric choices they should probably be human player only), some of them cumulatively can have significant impact on balance. I try to make the mods AI neutral in that they will only have effect when the COUNTRY mod is played as the human. I have just finished an Italian mod and again with really only one notable exception, all the events are well documented (or quite plausible), but the effect of even the historically correct events does make Italy stronger and changes the balance. Since play balance is something which is probably the hardest thing to get right and which causes (IMHO) the greatest angst vis a vis historical accuracy... 
Partly this is a result of dissimilar treatment for different countries (for example in relation to arms purchases or arms sales) but also because some of the 'costs' of certain actions/events are either not entirely or not at all brought out in the game - for example the diversion of IC to produce arms for export. I will hopefully get around to finishing the ai neutral version of the Italian mod and then I thought I would do an event chain for the Austro_Hungarian Empire.
I am more than happy for any aspects of my work to be integrated in CORE (and indeed to do the necessary coding - and I'm under no illusion as to how much work is involved there!) but in that case I would probably still make some submods to cater for the less historical events/make human play a little more fun/strengthen some countries.
Regards
Vermin
Partly this is a result of dissimilar treatment for different countries (for example in relation to arms purchases or arms sales) but also because some of the 'costs' of certain actions/events are either not entirely or not at all brought out in the game - for example the diversion of IC to produce arms for export. I will hopefully get around to finishing the ai neutral version of the Italian mod and then I thought I would do an event chain for the Austro_Hungarian Empire. I am more than happy for any aspects of my work to be integrated in CORE (and indeed to do the necessary coding - and I'm under no illusion as to how much work is involved there!) but in that case I would probably still make some submods to cater for the less historical events/make human play a little more fun/strengthen some countries.
Regards
Vermin