On the basic level, it doesn’t have as many integrations as slack. It’s mostly important for obscure services, as they often won’t be integrating thousands of chat apps, so they’ll choose Slack.
On the day to day level, it crashes even more than Slack, and syncs badly.
The pettiest and weirdest part is it doesn’t signal edited messages. The editing window is super short, but it still feels like being moonlighted when you read something and 2 secs later it’s a different thing.