The writing, acting, and direction/art direction are all wonderful.
As fiction it's very well & smartly constructed. As science, well, yes, it's nonsense.
I like this Doctor. I like him a lot. I wouldn't want to travel with him, but I love watching Capaldi do his thing.
I half expected the moon spiders to be related to the Racnoss, though.
While the spider FX was cool, I thought it was lame that they were really bacteria. Giant bacteria should look like giant bacteria. Giant spiders should really be giant spiders. It's like the FX department came up with one idea and the writing department wrote something else, and they just rolled with it.