Slack Infrastructure Engineer interview questions

The interview process consists of a coding challenge, phone screen with the hiring manager and then an onsite interview. I was given the option to skip the coding challenge and directly talk with the manager as I am a seasoned engineer. However, I chose to do the coding challenge so that I can have a good conversation during the onsite interview, if I were to be called. It took quite a bit of time to complete the coding challenge (> 20 hours) and given that I have a grueling full-time job, it felt like I was doing two jobs. Anyways, after the coding submission, within a few days, I was notified that I was selected for the next round and I had to talk with the hiring manager. The interview with the hiring manager lasted for 45 minutes and I felt like we had a very good conversation where we discussed my experience at length. Fast forward to a few days, I got a generic rejection letter stating that I didn't fit the bill. I honestly feel for the amount of effort I had to put in, I would have expected a bit more feedback beyond the generic "You are good, but not good enough" email. If you are interviewing for Slack, if possible, please avoid doing the coding interview. Or, talk with the hiring manager and explore a fit before you take up the coding challenge, that way, you at least know that working on the coding assignment is worth your time. Slack, If you are listening to the feedback over here, please understand that interviewing is a two-way street. If you expect people interviewing for your company to spend > 20 hours, we would expect a little bit of effort from your end as well.