Disallowing class action is absolutely in-favor of Slack and companies in general. Class action suits are a powerful pro-consumer force. They allow consumers to seek redress without cost - as the legal costs are paid for by the plaintiffs attorneys. Sometime consumers don't get much out of these suits while the attorneys make a lot. But their threat is a strong check on powerful companies.
Forcing arbitration is a murkier matter. The first google result on arbitration vs trial (http://www.nolo.com/legal-encyclopedia/arbitration-pros-cons...) provides a decent analysis. Frequently arbitration is less expensive for both sides because the process is simplified. You also avoid shenanigans like fighting for 6 months about jurisdiction and venue (which either side can do to seek a more favorable court).
The article states that arbitration is bad because consumers don't have legal representation. This is flat out wrong. Consumers can and should have legal representation in an arbitration. Just like in a trial, consumers need to pay for this legal representation themselves, and it would be foolish not to have it.
The article also says that arbitration is bad because companies have more legal resources than consumers. This is a fallacious argument. Companies will also have more legal resources in a trial. If anything, the simplified process of arbitration makes this less of an advantage. So arbitration may be slightly better for consumers in that respect.