Slack user groups are much less powerful than Discord roles. Slack user groups are _only_ a way of bulk notifying users. Discord roles control effectively both views of your server, and permissions (though why "Pin Messages" and "Delete Other User's Messages" are still combined into a single "Manage Messages" checkbox is a glaring flaw in that granularity).

And some other features like colours which are nice when you want mods to stand out for public servers, or teams for some sort of competitive event server.