>> It's just a matter of time before there's a huge incident.

I suppose that's correct. When (or maybe if, but probably when) Slack gets breached/hacked/owned it's going to be huge because a huge number of people are going to lose something that they didn't want to lose.

When I'm self hosting something and that thing gets breached/hacked/owned it's going to be huge for me because I and/or my company are going to lose something that we didn't want to lose.

I don't believe I can keep my stuff much safer than the big guys, though the point about Slack having a massive target is a good one. Maybe that makes it less secure?

I really don't know what's better for us in the case.