The only downside of this I can see is that your start-up develops a fundamental dependency (functionality and data storage) on a bunch of technologies owned and run by corporations you don’t control. You’re ceding control of your business (in a way you can’t easily shift somewhere else) before you even start. You could build a very similarly capable stack using open source tools that you could manage yourself (you do have capable tech people on your team, right?) for, say $100/month worth of commodity hosting infrastructure (that you could move around trivially if/when you want/need to). And why are people so enraptured by Slack? It’s just a messaging platform — there’re at least half a dozen similarly capable open source alternatives (I have a lot of experience with Rocket.Chat) that allow *all* the same capabilities, but without committing and entrusting your organisation’s historical communications to a proprietary platform you don’t control. Just saying.