Boilers, I felt were just awful. If an Alien dies, there will be acid blood spilled, why does there need to be a "L4D Boomer" type? The Alien does not have eyes, so calling it blind to begin with is redundant. So they only hunt by sound? Did the nasty crap that mutated them or got baked on ruin their other senses? Unfortunately, besides the chase scenes with the Raven, this was the only other nerve-wrecking part of the game that gave me the chills I wanted uniformly throughout the game. And after the first time, it wasn't scary anymore.
Spitters, I can live with. The design could have been a bit better, but I liked the idea that they were unique like certain kinds of snakes that spit venom. Better than the ones from AvP 2010 that were just dull grey in color.