All the upcoming ICO's and the ongoing ICO's stop using slack as the place for having your team. It's not worth. And also a big mistake.

What is wrong with Slack:

If you want to signup in slack, you have to go through a long process and every ico slack channel you want to join will have signup every time. It's not like having one account as every time you'll be asked to input your email and then add username, password. This can be more like frauds entering the channel with different names in different channels.

Open to hacking:

Strangely you can be hacked and compromised using slack. There are a bunch of people claiming to be the ICO bot and asking the users to send "xx" amount of eth to "XXXX" account. The ICO team have also said that it's not them and it's someone else. This calls for a real threat for the users and the possibility to be hacked easily.

The Bot Spam:

Yes! There are a bunch of bots that keep spamming you forever. Even if you mute them, there will be the other hackers bot pinning you every time. And this is really annoying to the users. (You need to understand this ICO's)

What are you'll missing on By not being on Discord or Telegram:

Discord and telegram are used more than Slack. There are more users to Discord and Telegram than Slack. Telegram is more easy and convenient to use. It is also safer to use telegram compared to Slack. (If you had 1k Slack members, out of that only 100 would be active.) But in Telegram, there is almost everyone active.

Discord is one of the best places you can be in order to promote your ICO. Because there are a huge number of users joining Discord every day. There are a lot of Crypto users in Discord too. You can reach them easily compare to Slack. Also, the User Interface of Discord is far superior to Slack.
You can set up your own bots or use the existing Discord bots. They are much better than what is in Slack. You can set up rankings for the users and also have voice conversations and much more.

By not being on Discord, you are mission out on the potential Investors. And I believe that is not your intentions.