In my experience, slack has a poor signal to noise ratio. Important information gets lost as people post gifs, memes, or banter in the same channel.

Search has gotten better, but it's still not great.

Some people expect you to respond immediately, defeating the purpose of asynchronous communication for non important / non blocking matters.

The format (like IRC) encourages multiple fragmented sentences instead of one long, cohesive message.

Slack isn't that bad by itself; it's mostly how people use it compared to say, email.