Kingmaker is kind of the opposite of Underrail, combat/difficulty is largerly braindead (although it's systems are fine), but dialogues and C&C are pretty good, so pick your poison.
Haven't tried PF:K, but UR dialogues are excellent and C&C is pretty good (although might still leave much to be desired for some people), especially in Expedition.
it's ok. New weapons, psionics and feats are neat, but the actual expedition campaign is way worse than the base game.
it creates more questions that it answers.
Not really. The Black Sea is just Deep Caverns 2.0, but with botched encounter designs, more obnoxious enemies, and no respite like the hatch in Hollow Earth. The rest of the stuff is cool, but the aforementioned bad might likely turn you off.
Only if you actually cared about the story in the first place, and if you've actively involved yourself with the ((((((Secret Society))))))™ of Underrail regarding the supernatural elements of the setting. You'll get answers in relation to that, but when it comes to the history of Underrail itself you'll be leaving the Black Sea with more questions which prompt you to ask Styg, "When will we visit North Underrail?"
Not really, unless you meant with the recent nerf of having them came out of the hives every 2 turns instead of every 1 turn. Still, it's dumb not to have their entire swarms eliminated with a single TNT charge.
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