Mattermost and Jitsi are the most enterprise grade open-source alternatives (when properly integrated) to replace Slack completely, except for the plugin ecosystems. Many large companies have done that switch to opensource successfully. However, for SMB's such implementation becomes extremely complex to set up and maintain.

Noysi here has done a fabulous job of launching that as a solution for everyone out there, whether large company or small. They have added major improvements to mattermost such as adding a task manager among other things. They have also done a great job of integrating jitsi so well that all channel meetings can be recorded and shared immediately within channel. This is unlimited online recording storage 🙌 and immediate sharing of recording with participants.

And that's the other thing. Opensource allows you to have complete control without any paywall based limitations. A SaaS typically does not give all controls to you and also implements limitations per membership levels. This Noysi offer in Appsumo is one of those very rare SaaS offers that has no limitations at all.

The only thing you can do more in case of open source mattermost is to leverage a wider plugin ecosystem than the one Noysi offers you. But Noysi has made so much improvement over mattermost, that it overshadows that one issue.

Hence, the product is as robust as it can get (years of development), and the company which is hosting/providing the service seems to be on solid footing as well as per their crunchbase profile.

If you are looking to replace slack, there is not a better deal than this anywhere right now, and I doubt if any can ever be made better than this.