Curious what you think an alternative is for enterprises. You can self host rocket chat, sure, but sometimes you just can’t afford a devops team just for internal tools. Slack is cheap and easy.
I've recently changed jobs and my new company uses GitHub for everything, including "projects", its wannabe kanban boards, and I'm truly appalled at how bad they are. They've been out for many years, yet they lack the most basic features, like a decent API, or hierarchical relationships between tickets, or a simple integration between the board and the ticket status... Like what are they thinking!?