They also become much more attractive targets for APT since compromising say Slack is way more interesting than some avg. mid size shop.

I think you're right in general, but it's also worth noting that on-premise software stacks are still very attractive targets; companies may be running old versions with vulnerabilities, so attacks are both easier and can continue for longer--one site patched the vuln? On to the next one; just move down the "customers that use us" list on the provider's sales page.

On premise deployed stacks are usually shielded off by network firewalls though, which cannot be said about most cloud based services (yes, I get it from a convenience point of view).

In order to attack an on premise application that is safely behind a firewall in a private LAN, one would have to come up with more creative, staged attacks (such as DNS rebinding or blind XSS attacks).

So, put simply, whereas the attack vector of a cloud based service is basically "the internet", this isn't usually the case with on premise deployments, and therefore they are inherently safer, or at least much harder to attack.

On top of that, cloud based services are usually multi tenant environments, so the assets to gain there are huge. One serious hole in such a service (be it Slack, logz.io, Github private repos, New Relic, etc.) will probably be a disaster.

It depends on what an attacker is after.

For the average mid-sized shop that's probably true. However, it's usually larger organizations that worry about storing data in the cloud.

For these organizations and attacks specifically targeted at one of them, I suppose that in order to access vital company data both social engineering and hacking an internal system are much more effective attack vectors than gaining access to a third party system.

Depends if APT is say FSB or MSS Slack is very attractive target. But in general you are prob. right.

Yeah but this goes both ways; Slack and Microsoft know they're huge targets, and have competetent detection and mitigation in place to deal with intrusions. Of course that doesn't mean a breach won't happen, but it's not like APT casts the "hack" spell and suddenly all your PII is out in the open, you have to fuck up somewhere and not notice for a period of time.