When most people read "hostile creatures", they probably thought of mobs that would attack you first. However, the actual implementation just checks for mobs with the "hostile" tag, when a significant amount of mobs don't have it—which I'm guessing was unforeseen.
Merms, beefalo in heat, pig guards, killer bees, enraged moslings, even—wait for it—ewecus, which is a particular favorite for spawn trapping in my experience. Not to mention specific cases for monster characters/Wurt/Wigfrid as well as spawners like cave entrances.
The current implementation does protect against some common killers like tentacles/eyeplants/spiders/hounds (not to mention the separate blocker check), but it could be much better. The design idea itself seems pretty bulletproof so far, so hats off to the devs for that one, hopefully it stays that way.