The following discussion contains massive spoilers for Fallout 4 and the Far Harbor expansion. You have been warned. The spoiler-free conclusion is: I’m not particularly fond of Far Harbor. Now, on to the spoilers… (after the screencaps)
The screenshots above illustrate the big sticking point of Far Harbor. (Click/tap either image for the full 1080p version.) The shot on the left is Boston (aka the Commonwealth), looking toward Bunker Hill. The shot on the right is the island of Far Harbor from the Acadia observatory/synth settlement. That fog you see in Far Harbor is the main story element, and it’s ridiculous. When you’re down in it, you can’t see where you’re going even during the middle of the day. Forget about wandering around at night, you’ll likely drown or fall off a cliff.
Even after spending a dozen hours or so working on this adventure, I have no real idea what anything looks like. Fallout 4 is a beautiful game, but you wouldn’t know it from this expansion. And, no, it doesn’t heighten the suspense or make the island more dangerous. It’s just annoying.
I’ve seen at least one game site writer wax rhapsodic over a “twist” presented early in the story: is the main character really a synth? Who knows! Maybe! It’s up to you (the player) to decide!
It’s ridiculous. It’s established in the main game that you are fully human when you go into cryo storage because your fully human son is kidnapped to supply genetic material for the look-like-human synths. (I.e. such synths don’t exist pre-war, so you have to be human at that time.) The only way the Sole Survivor could have been replaced by a synth is if your own son did so! Which is patently ridiculous. He has no reason to do so and then drop you back in to cryo; he would just place you in the Institute with himself with no knowledge of what the world outside looks like.
The rest of the story is fairly interesting, but it’s wrecked by…
They’re jerks. All of them. (Well, maybe not some of the people in Far Harbor, the town.) Within a few minutes of meeting DiMA I wanted to shut him down and ship him back to the Institute. (I was playing with my character that had sided with and become director of the Institute.) And that was before I played the memory-bank puzzle games and found out just how much of a dystopian dictator the little weasel is. I had Nick Valentine with me and he was right to dismiss any talk of himself and DiMA as “brothers.”
The Children of Atom are their usual fanatical selves, which is not attractive. What was somewhat distressing was suddenly, their little religion seems to have some basis in fact. You go on a vision-quest and actually have a meaningful vision and discover an artifact, and all kinds of cool stuff! Except, thus far in the Fallout universe, the CoA are just crazies. I wanted to treat them like crazies—and, they still act crazy—but my character is supposed to take them seriously and maybe join them on their little quest to return the entire island to Atom! Yeah, right. Where’s that nuclear launch keypad, again?
The townsfolk in Far Harbor are jerks, but they’re my kind of jerks. Just rude, and occasionally duplicitous, but mostly just trying to survive in a very inhospitable environment.
Lastly, like any grand game from Bethesda, there are bugs. This time, though, they are game-breaking. As director of the Institute, I wanted to reset all the synths and send them back. Can’t do that, you can only subdue them with force. But that doesn’t work, because they keep getting back up and have to be subdued again, even though the game says they’re all ready for processing. The doctor from the Institute never reappears and there’s an endless cycle of shooting all the synths and then shooting them again. I finally gave up and tried something else.
That something else was to confront DiMA about murdering the mayor of the town, but he didn’t want to talk about it. So, I went to the town and outed the mayor as a synth and then talked the townsfolk out of killing her, which they promptly did as soon as I finished convincing them not to.
I’m going back to the Commonwealth and continuing work on my global empire.
 If you do play this expansion, get the first level of the Aquagirl/Aquaboy perk right away. You’ll need it.
 The constant radiation isn’t a problem unless you try going to the island at very low level. A suit of power armor and a dozen or so fusion cores will keep the radiation from becoming a problem.
 Not recommended, even though the game imposes no level requirement on accessing the content.