To post my thoughts from the discord to the thread:
Alliance is the best option for the UF at this point in time. The RA has consistently stated that they don't wish for a full merger, and both the merger and break alliance options inflict a major debuff on us, at a point when we need every available bonus to avoid losing our gains.
While the break alliance bonuses are tempting, it turns the war from a 20 v 24 (order-wise) into an 18 v 18 v 24, which is worse odds for us, especially since the RA will likely choose to fight to the death to spite any betrayal rather than succumb to any initial decapitation strike. Even what theoretical upgraded traits we can get aren't worth a two-front war, not with the NC being such a massive threat (seriously, 1 million, 800 thousand troops under arms, around, with more on the way, even after they just raised 500K militia.).