|While I use Slack myself, Stallman have a few issues with it.|
Granted, Stallman is a bit over the top regarding Security and Privacy.
That said; I provide you with his thoughts;
Reasons not to use Slack
Mattermost is a Free Software alternative to Slack.
The Slack client (desktop, web, and mobile) is nonfree software.
Slack centrally stores all postings permanently; the users have no choice.
The Slack's API TOS forbid replicating the product or service using the API would would seem to prohibit making a free software client for Slack.
The Slack's API TOS can be changed arbitrarily by Slack.
The Slack's API TOS prohibits deriving 'know-how' which would seem to prevent a free software developer who agrees to the TOS from working on future Slack competitors.
Slack can and does determine the user's geolocation, and uses that to block connection from Iran and some other places where the US has imposed sanctions.
Slack initially provided gateways to allow connecting from IRC or XMPP. Eventually Slack turned these gateways off, providing no ways for users who were accustomed to using standard protocols and Free Software to connect any longer.
There is no offline or local archive of Slack messages and no way to retrieve old messages if your account is cut off.
By default Slack fetches a copy of and scans all URLs you include in your messages.
Slack centralizes communications in unencrypted form.