I had the opposite experience. A client of ours was souring on us and we invited them to our Slack to try and help them with some issues. Slack was the easiest

I had the opposite experience. A client of ours was souring on us and we invited them to our Slack to try and help them with some issues. Slack was the easiest way to help work through their issues. The ability to quickly and fluidly clarify ideas in a conversational manner as a discussion is ongoing make it far superior to email for technical conversations. Often in emails people seem to talk past each other, as there is too much time between emails and people lose track of the conversation too easily. Video calling might be even more efficient during the call itself but personally I would spend so much time thinking about the call, preparing or worrying before the call itself that it would be a bigger drain on my productivity overall. Not to mention that my personal productivity is not even the most important thing. In this case these Slack conversations fixed a client relationship and moved our work with them forward in a way that was far more valuable than any SLOC I could have produced in that time.