Fear Island isn't a great movie, and that mainly stems from the fact that from the beginning, as an audience, we can sort of see where it's going.
Unreliable narrators are sometimes fantastic (The Usual Suspects comes to mind), and sometimes not so much, because there comes a point in which something is overdone, and it loses what it otherwise could have had. I feel that happened here
The story isn't that bad: A girl is found on an island with six bodies, and she recounts her tale to the police, who are at first suspicious, but grow to accept what she says. And we have twists throughout. Or maybe there aren't, as unreliable narrators can leave things out, lie, etc.
I liked how this movie was set up. But it was made past the time in which these types of movies weren't uncommon. Hell, it's moderately similar to The Hole, and that came out way back in 2001. So no, Fear Island's not great. Some potential inconsistencies, almost no gore, somewhat annoying characters, idiotic characters, and unnecessary twists. You could do worse, though, and despite the problems, it's certainly a movie that tried. Less than average, pretty generic even, but not disastrously so.