This film will find its audience among fans of dark comedy. Don't expect the broad appeal of "Shaun of the Dead" or "Hot Fuzz", and comparisons to other Simon Pegg films miss the point. This is a film deserving attention, especially for writers and the "cult" audience who enjoy quirky, unique dark comedies/mysteries.
Reviewers forget that some films are not for everyone, and their opinion is worth nothing in the evaluation of such films. This one may be very much enjoyed by a smaller audience, even after several viewings. "Fantastic Fear" ranks among the "love it or leave it" types; you may find it annoying or too odd, like "Dark Shadows", or it's tremendous fun.
"Burke & Hare" also divided audiences, yet this film has more in common with murder-mysteries, like a cross between "Clue" and a spoofy take on say, the recent "The Raven (2012)". Better than both films, IMO, this will live on with DVD and Blu-ray releases, thankfully.
Box office numbers and critical "consensus" are ultimately short-lived, and very deceptive. Give this a chance if you like dark comedies. And it must be said that Simon Pegg's acting resume becomes increasingly impressive. Regardless of the final product, you have to give him props. And here, the final product is better than you might expect.