Also, one of our main USP is the ability to take action on any notification. Slack provides a strong api layer to pull this off, while its extremely tough to do the same as a gmail app. Chrome extension is still a viable option, but for now, we chose to go ahead with Slack due to the collaboration element.
Regarding the engineering manager usecase, although we don't have deep relevant integrations yet, we are considering adding support for Jira, Trello and Sheets as well, but would love to know if any of that will be as helpful in your day to day work. We are not venturing into tech alerts yet, as there are good comprehensive solutions already in place but would love to know of any gaps that might exist where a configurable action layer would be helpful.