One UI nit:
I clicked on Mathematics and the top submission at the time was: https://ninfex.com/item?id=sVfpxepq4rNQ
The search UI says: "5 days ago."
I thought it sounded odd since I missed the discussion here 5 days ago and clicked through to discover the link leads to an HN discussion from 2014.
So, that "5 days ago" refers to when it was "submitted" to Ninfex instead of any relevant time information on the actual submitted material.
Felt a bit misled, and finding relevant current information over higher ranked older material is a painpoint for me in other search products.
Happens when I am searching for information on a library and the most popular hits refer to information from years ago and the library from multiple major versions ago. Sometimes the answers are incompatible with the current state of the library and its API.
These are different issues, but they feel orthogonal.