That's surprising because if you pick up any professionally typeset book or magazine, you'll find ligatures all over. Some of them are quite subtle (e.g., the fi -> fi ligature). I have to imagine then that your colleagues are getting distracted not by the presence of ligatures in general, but by a few decorative ligatures or characters present in the Slack font, which looks to be Lato. For example, Lato's ampersand is missing a chunk in the upper loop (https://www.google.com/fonts/specimen/Lato).