For what it's worth, I think the key failing of the "zombie thing" - regardless of the fact that it's written in an intentionally more subdued, second-key, factor than the previews suggested - is that the choreography is kind of a letdown. The episode succumbs to the classic watch-'em-tumble-through-a-window trope, for one thing, and that only ever really works when there's a great deal of scale involved. Instead, it was seven or eight of the things shambling from the sill, on to the floor, and then standing back up.
What's more, the same trick gets repeated when the force field gets temporarily lowered. Even with the increased scale here, it doesn't quite work; we're now seeing a metric frakton of the little bastards, but only one little puncture hole where they're getting in. Which, to be fair, is precisely what the narrative requires here, and that's fine! Again, it simply does not look good to me.
It's little things like that. The darkness-fueled introductory scene with them struck me as suitably creepy, but from there on out, it was kind of like waiting around for Bytha's henchmen to eventually get bitten, shrugging, and moving on.