Everytime an engineer tries to push slack on a consultant all they see is a new interface, shit loads of noise, gifs etc (cultural issue rather than tech stack obviously) and no real extra benefit vs the cost of learning something new when what you have today ticks all the boxes.

I also use both as I'm a partner on the engineering side of the business but also run a large consulting pod - I really don't care which one anyone uses. I don't personally see Slack being better 'enough' to justify the effort of switching for those that are already on teams.