Low chances of that mission success and the fact that the targeted country may get a CB against you and declare a war. That war would call his allies. Because of Alliance CB, allies of those allies might also join in, especialy if some of those allies are your rivals.
And add the high infamy cost in case your spy is detected and you may regret your immoral actions.
Except, the Casus Belli given against that improves Prestige and revocation of cores, it's a pretty poor Casus Belli by all standards. The infamy if I fail is irrelevant, since if i'm using a tactic of forcing PUs, I don't need to really take much land anyway. If they use the CB to DoW, then think about this. I wanted to get involved in a war anyway, so I know I can beat them and their allies. Hence, I can pretty much get them to concede defeat or white peace out and simply try again.
Whereas, If I succeed, I can then proceed to subjugate an entire nation, irrelevant of size, into a union.
The only ways to balance this appear to be:
-That one particular spy mission has a huge cost, proportional to the power of that nation. (Manufacturing a prominent claim on the Kingdom of France should surely be much harder than a HRE Duchy, at least harder to get one credible enough to warrant this).
-If you fail that mission, there's grave consequences. People now know you'll forge documents on other thrones. This should give infamy & reduce your own heir's credibility (after all, if they'd do this to another throne, what's to say their claim to their own isn't completely manufactured?).
-The mission should probably also take multiple spies aswell as have a lower chance of success It's a big task.
Another option is that nations could then contest this Union in someway via another CB forcing you to end the union. But as it stands, it appears at a glance to be immensly overpowered. Because it invites a whole new playstyle. Doing absolutely nothing except making money, getting into wars via this and forming unions over the major powers. Then it wouldn't be long before you could go berserk, considering every power that could feasibly stop you is either in a PU with you, or has been inherited.
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