Here's my reasoning: this is a wargame, and if taking Minors at the start is a matter of just saving a couple of turns of PP and playing a card, then you're essentially removing any incentive to engage in the game's strongest feature: combat.
It's not quite as easy as you put it. First, you're gonna need a foreign affairs council. You need a good diplomat or some shadow diplomat stratagems. You need to make the regime a protectorate, then a vassal to reduce difficulty. Hopefully with critical successes to improve your relation with them. If you fail those and reduce your relation, you're gonna make the unification harder, and I think you can't really increase your relation with them, unless you have that one political faction that gifits you relation boosting stratagems. It also depends a lot on the tradition value from the minor, which as far as I can tell, can be a +200 modifier for the difficulty at max tradition.
When you do succeed though, you're immediately getting access to their city with ecstatic population happiness and all their militia units, which both are really powerful effects.
I would actually say that the "annexation" card is currently underpowered, because who would ever want to play that when there's a risk of losing everything you worked for diplomatically and dropping right down to a war status?
It doesn't really make sense for unification to have -50 difficulty, when it has no negative downside for failing it. The -50 difficulty should maybe be moved over to annexation.
I'm okay with increasing PP costs, since the game feels like it needs more PP drains in general.
There's currently no effect for critical successes/failures with these cards. The annexation war declaration should probably be moved to critical failure since a normal failure could be seen as the minor faction challenging your threat and it still being your choice if you really want to declare war or not. Normal failure should still be a big drop in relation between you and them though.
Maybe the ecstatic population happiness after unifying should be an effect of a critical unification success. Normal unification success should be 50% happiness or something like that.
although I do wonder if just one of them might do since they're both very similar?
The game is parts roleplaying game, so having two different options for unification/annexation, one for intimidating with threats and the other for being diplomatic, is good I think.
The cards could be made more different from each other for sure though.
One of the guys on our discord has already shown us how he's easily taken large amounts of territory and thinks it's overpowered.
That was me. It's very powerful and it does need some tweaking, but I don't share the worry of it potentially being game ruining.