That the cloud service company had access to your communications was something that was brought up at Blekko which was using Google's hosted domain feature. On the down side, there was at least one documented case of an engineer pawing through other people's mail. On the other hand, a lot of the VCs and partners had Google hosted email as well so "copies" of our communication would be there anyway.

At the end of the day the vendor's priorities were aligned with ours, they want to sell hosted mail/docs/etc service and having people with bad experience (even one) really limits them. So I think Slack would also be similarly aligned with maintaining their security and the security of their clients for exactly similar reasons.

The issue that Schneir pointed out about protecting your data when you're SaaS provider goes chapter 7 on you, that one is still real. And while there are people with cash hoards who could do it, I've yet to see someone spend money to buy a SaaS company so that they could get access to the data of a competitor. Although to be honest I did wonder a bit about Yammer. Killing a potential competitor (Whatsup, Instagram) sure, but actual industrial espionage would seem to be an unlikely outcome. (Would make for a fun story though, if you're reading Charlie :-)