I’ve seen this get raked a few times. Slack is a product, and they have decided to provide the best experience in their apps, then Chrome browser, then everything else. I use the macOS app, can’t see advantage of using browser. None.
It requires significant effort for us to build out support and triage issues on each browser, so we're focused on providing a great experience in Chrome and our desktop apps. We are listening to all feedback, though. 👂