Umm... why do people always assume "hosting it yourself" is more secure and not less? Do you have Slack's security expertise and budget? In my experience when small to mid-size companies attempt to manage security themselves they do a passable job but are convinced they are doing an excellent job - until they get hacked.
Larger companies usually have the budget, tools and expertise. But even then there are lots big companies with mediocre security too.
Larger companies usually have the budget, tools and expertise. But even then there are lots big companies with mediocre security too.