: Sci-fi horror is at its best when it explores the unknown. A 3.9 rating often happens when a story explains too much. When a writer spends 20 minutes explaining the quantum physics of a monster instead of letting the audience's imagination fill in the blanks, the "horror" evaporates into a dry lecture.
: In horror, atmosphere can sometimes save a weak plot, but in sci-fi, audiences expect a certain level of internal logic. A 3.9 often indicates a "Logic Breach"—where characters with PhDs make decisions that would embarrass a toddler just to force the plot into a scary scene.
Here is an "interesting write-up" on why these mediocre ratings happen and how to spot the hidden gems within them. The "3.9 Anatomy": Why Sci-Fi Horror Often Flops