That depends on your definition of secure!
Touting "industry standards" and saying you take security "seriously" is one thing, but building a product from the ground up with no purposely designed backdoors is another entirely.
In short, Slack employees have access to every message and file you share on their platform. Slack's security is based on methodologies extremely similar to email and other common messaging apps like Skype, which, as high profile hacks over the last few years have shown us, are not very secure.
Additionally, Slack has previously suffered numerous significant security breaches. For individuals or companies concerned with privacy and security, this may be unacceptable.
For more information on the security pitfalls of many popular messaging, email and file sharing apps, you can review this blog article.
To directly compare HighSide to competing apps such as Slack and HipChat, view our comparison page.
The choice is yours!
Note: You may also find this article interesting on why HighSide does not offer a web-based app like many other chat apps (including Slack/HipChat) do. Hint: because it's disastrous from a security standpoint.