I just finished Far Harbor for the first time and I'm about to get started on Automatron
I liked the quests and the story but the island was such an awful place to get around, my least favorite parts of the commonwealth have always been those areas where it's just kind of...lumpy? and that's the majority of the far harbor landscape with the addition of fallen trees everywhere and constant fog and murkiness that makes it a pain to see much of anything, it was actually making my eyes hurt
the Red Death quest and the story with the bowling alley were top notch though, I'm afraid I may have missed some other locations with good narratives but I hate moving around that island so god-damn much that I've got to at least take a break before I consider looking around any more
I rather liked Far Harbor, but I do agree with you regarding the annoying terrain.
The most annoying aspect though is that all 3 of its player settlements have the same poor terrain and it can be quite tricky to build decently on them.
One of the settlements is also mostly a pre-existing building to work in and as with most of these the default settlement system struggles as you try to squeeze things in without the game telling you they're clipping.
As for interesting Far Harbor locales there's one on a small cluster of islets in the West (If my memory serves me) which has a decent self-contained story, and it has a pretty good reward for any crafters/settlement builders, which also periodically respawns.