This video sums it up pretty nicely imo:

https://www.youtube.com/watch?v=_mqVnyOTeN8

TLDW:

- Nix needs weird tricks to make its packaging scheme work

- The main repo is uncurated and hosted on Github

- The functional nature of packages means a low level update can either trigger a huge wave of updates or you end up with out of date deps. Usually both happen at the same time