I've never used Slack but the usability of Teams is awful. So much empty space, empty context menus, usually only one way to get anywhere. Notifications for a user can only be disabled by context menu, clicking away a notification doesn't mark the message as read, all settings are on a profile basis (no way to disable notifications for mobile only), there's no way to just stop Teams on mobile without having to sign out. The chat input editor is horrible when it comes to writing / pasting code.

Video chat and calendar integration work great, though.