Well for its worth my experience interviewing for the data team there was terrible. A long coding exercise that when submitted resulted in a 7 day wait and a 2 liner email. Wouldn't recommend.
What surprises me the most about the Slack's job page is that most — if not all — the positions are on-site. It surprises me because most of the companies that I know are remote-friendly use Slack as their main communication method, so I would expect Slack itself to have some remote positions just for the Dogfooding [1]. I have applied 3 times for a regular SDE position there and two times I was rejected because I was not (permanently) living in the US, the 3rd time I got no response while staying in NYC.
Which raises the question: With such a good tool for team communication, why does Slack need an office? Why not do all your work virtually?
Slack CEO Stewart Butterfield gives product manager Mat Mullen advice, and a ukulele serenade. “There are some conversations that are much easier in person,” says Brady Archambo, Slack’s head of iOS engineering.
[1] https://en.wikipedia.org/wiki/Eating_your_own_dog_food