The B-M themselves are bad enough, until you realize that their teeth (which are all molars) don't rip off chunks of flesh from their victims, like say, a piranha. They can only latch on, and chew them to death, meaning that you feel every single second of them eating you, up until they swallow whatever piece of you they succeed in chewing off, and they eat everything. If you're attacked by a huge swarm, quick death. However, when you're only attacked by a small handful, your death will be an agonizingly slow. Worse, either due to shock, or some manner of numbing effect from their bites, most victims don't initially seem to feel pain from the first few bites, so many victims only seem to realize something is wrong until they look down and see a B-M, happily chewing away on their sweet, sweet flesh.
And then we have America's version, which can infiltrate just about any space, including you, and pull a Chest Burster once you cave into your need for water. Oh, and it's a tentacle monster.
Furthermore, Shingo, when he feels like it. Holding a gun to his face? Fine. He'll give you a perfectly calm death smile, and terrify you into surrender.
There's a good amount of horror actually, and notable in that, for the most part, it manages to creep the Dickens out of you without resorting to anything graphic. The ending to the chapter Contract, for example, ranks as one of the most horrific moments in the book.
The beginning of the second outbreak. Just imagine you being saved in a lockdown building from tentacled monster, that grows before your eyes, by a bunch of soldiers, and few hours later, the same soldiers come back to BURN YOU ALIVE for witnessing it.