For work, the decision of which IM tool is most likely going to be a corporate one, without much choice of solution. I guess the problem here is the same as for other corporate tools. (And people have strong feelings about corporate tools, but it seems there are less strong feelings about "using jira vs bugzilla" as there are "using slack vs irc". Or maybe there are as much strong feelings.)
On the other end, I get the troubles with the "network" effect for out-of-work communities (open source projects, clubs, etc...) Such communities can at least have "some" level of debate about which solution to use, and balance the pros and cons as they see fit. It's completely acceptable for an OSS community to discourage using such platforms - as any other non-free, centralized platforms. We're still not forced to use anything.
Then, there is the "club of non-power user" situation. I would be hard pressed to ask someone from my drama club to setup and maintain an IRC server (neither do I want to do it.)
Thanks for adding this dimension.
Follow-up: is there a "nice" provider of some slack-like solution (mattermost, etc...) that hosts servers for communities of non-techies for not too much money ?
For some definitions of "just" and "working". Even ignoring the business limitation, the UX of Slack search is meh. That said, I'm yet to see any of the modern web&mobile-oriented IMs that would have a decent search UX.
But yeah, your list is solid. They offer plenty of value, just mixed with plenty of negatives.
A big source of the complaints against Slack is usually the network effect - in context of both work and OSS communities, it's usually somebody else that makes the decision and imposes it on you, forcing you to run face-first into all the "legitimate reasons to not use it" you listed. Half of the pain would be gone if they opened up their service to third-party clients; I'd say the reason they didn't have as much opposition from people initially is because they run the bait-and-switch with IRC gateway.