Earlier quoted context omitted.
"While Slack offers many benefits to customers, there are also downsides to using the platform, including high subscription fees and the risk of a massive leak of private data if Slack’s servers are ever breached (again)." Not everybody wants to pay to put their potentially sensitive data onto somebody else's systems, where they can't control the security of that data.
The one thing in terms of security that is even a blip on my radar is that slack does not encrypt data at rest (disks). [1]( https://twitter.com/slackhq/status/467476452364279808 ) Their reason that they couldn't search through data if it were encrypted is not true. They are using AWS, and can simply use EBS encrypted volumes ( http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/EBSEncryp... ). Should be an easy flag…
Both Slack and Amazon can steal all the data without being detected, regardless of whether the servers are encrypting it or not, since if the server can decrypt it, so can Amazon (just scan VM memory for the key from the hypervisor).