For what it's worth, security is tangentially related to my job, but not my focus. I do deal with data storage and protection, though, so I have at least some attachment to this area.
One of the problems with Chinese ownership of technology platforms is that as a Chinese citizen you are required by law to give up any data or information to what is basically Military Intelligence. That isn't just what you, yourself, own or create but anything that you have access to. Because Bytedance is a Chinese company that employs Chinese citizens, any data that the intelligence community in China wants is required to be handed over to them.
Bytedance has tried to say that because TikTok has their own US operations, there is a firewall between US data and China. That doesn't really track, though, because if they have just one Chinese citizen with access to the data - which they do - that data can be assumed to be in the hands of Chinese intelligence.
The most common objections are (A) So what? I'm not that interesting anyway. And (B) but so many other companies are gathering data anyway so who cares?
Taking the second point first: yes. That is true, there is a ton of unethical data harvesting going on and the impact of it is yet to be seen. That doesn't mean that we should give up.
For the first point, when you have very large sets of data you can start to tease out things from the "uninteresting". Quantity really does have a quality all its own in this case. China is spending a lot of resources to collect enormous amounts of data because they know that data is not necessarily time-limited. They're amassing encrypted data and storing it in anticipation of advances in quantum computing that will render most of today's encryption as secure as the Enigma. It does them no good now, but collecting it and storing it is relatively cheap so they consider it an investment.
Using the app to cull data from your phone could enable data scientists to look for patterns that would indicate military movements or the routines of people in sensitive positions. Think about how the political world reacts when there's a rumor that someone running for high office said or did something that might offend someone 30 years ago. What if a 13 year old kid now said something in private or in a posting that they later deleted. But in 20 years they want to be a senator and a member of the Chinese intelligence service shows up at their house with a video of them saying something that is innocent now, but highly offensive in 2 decades. Basically, there are so many things that can be done with a data lake of that size that we can sometimes not even imagine what can be gleaned. As computing power increases and as machine learning gets further developed, we can get more data out of throwing data into the Wonkulator and just asking it what correlations it can find for us.
Honestly, that should give pause to people no matter where that data is held. But, in theory at least, if it's in the US there should be some guide rails around it. And at the very least if the government is demanding access to the data it's our government and not a foreign power. Not a big comfort, but marginally better maybe?