Pros
Overall, Slack is a wonderful place to work, to be an engineer, or to be an engineering manager. I've been lucky enough to be both an IC and an EM at Slack, and over the past two years I've worked with incredible colleagues. We're shipping a well loved product that's fun to use, and we're able to ship it without working crazy hours. Work life balance is great, and so are benefits. During the pandemic they've been giving everyone in the company 1 extra day off a month and it's just been wonderful.
Cons
If you don't have a manager invested in helping you grow your career, it's going to be a challenge to do it. The engineering promotion process is very manager driven, including packets written by the manager. If you don't have the proper support, it's tough to get promotions or switch from IC to EM. If you do, it's a great place to grow your career. On a day to day basis, there are still a number of things that cause engineers on my team pain in the codebase. Flaky tests are probably culprit number one. Code health has improved a lot in the past year, but there's still a ways to go.