Yeah, IRC clients are hard to use, too many are command-line based, although on Linux you can still find a few with GUI, but not as pretty as Adium on Mac OSX. Your mileage may be different, but when I was working at Mozilla everyone used IRC and it was not a huge burden because most people really just do /join or pm a person. As long as you provide a documentation on how to use IRC, you are golden for the most part. If you mandate people to use IRC and have resource available to help troubleshoot or setting up IRC, you are good. If you need to set up some complicated things with IRC, well, it is tough, but you are on your own.
Slack's interface is not very impressive, very messy in my opinion, but that's me. There are businesses out there offer IRC as a service, so that's another option.
Final point: I really don't care about FOSS vs OSS vs Proprietary. As long as the company truly respects my data privacy and security, and is easy to use, I can give zero damn about either of three. It's 2015, we need to stop arguing and actually make things better. Business needs to focus on improving product experience and security. But you know what, some people do, that's fine, none of my business anyway.
Slack's interface is not very impressive, very messy in my opinion, but that's me. There are businesses out there offer IRC as a service, so that's another option.
Final point: I really don't care about FOSS vs OSS vs Proprietary. As long as the company truly respects my data privacy and security, and is easy to use, I can give zero damn about either of three. It's 2015, we need to stop arguing and actually make things better. Business needs to focus on improving product experience and security. But you know what, some people do, that's fine, none of my business anyway.