It is hard to express tone through text, even between people who know each other well. Situations worsen depressingly quickly, it's too easy to feel offended.

From experience, I refrain from giving any negative feedback through Slack and go speak face to face. I also watch out for any misunderstanding lasting longer than a few minutes, even if it's between others people.

Hedonic index increased tremendously at work in a few weeks when co-workers started doing the same.