What is the issue with TikTok?

If you have a tiktok account go back and read the terms of service that you have agreed to when signing up. They own your content, not you. Additionally they have access to all your photos, videos, media, contacts, location, location history, emails, messages, calls, message and call history, web browsing history and all your apps. Through the app they can remotely observe where you are, who you are contacting, and what you are discussing. You have agreed to allow 24 hour surveillance by a foreign government. You just put the novel 1984's Big Brother in your pocket or purse. This has been a concern long before a bunch of teens and 20's sabotaged a rally. People on Reddit have been expressing concerns about this for a while.

The issue was that it does not contribute anything good to our society. 

Privacy concerns in terms of sharing private information of users I gather from what the American president says.

TikTok is where they came up with the idea to flood Trump's Tulsa campaign with millions of ticket requests.