I'm a part of a ton of Slack communities but I'm active in only a handful.
Slack works to its fullest only when it is implemented in a team and everyone's supposed to use it for communication. Slack could/should improve on making the communities on it better by adding relevant features.
Easiest way to get everything under one roof.
Building active communities is really hard.
As a remote designer, this is a must have. I'm always contactable and available. Super easy way to also have quick calls and communicate with other team members on different teams.
Possibility to be logged on multiple teams, being available at all times on mobile and desktop, and easy to connect with other services.
I'm pretty happy with Slack, so no cons for now :)