To be clear, TikTok and WeChat are quite different 'threats', both real, but it's not clear how dangerous:
TikTok is aimed at a wide market, and it's a content app - there are two worries with it, data and influence. If they are installed on a significant number of devices in the US/the 'West', and their user data is stored in or available to Chinese networks, that data is available to the Chinese government on-demand. Secondly, if they have significant enough cultural influence, they can use that to promote/demote certain topics or specific content, for a variety of purposes.
WeChat seems to have no serious ambitions beyond China and the Chinese diaspora, nowadays. It's used by people all over the world to keep in touch with their friends and families, and to do business, with people in China, and it's widely-known that the content is actively monitored and censored by/on behalf of the Chinese government. Although VPNs etc are available, it's really the only significant communication and social media platform available in (and into) China. There's also pretty good evidence (although mostly anecdotal, AFAIK) that it's a channel for nationalistic propaganda, government-coordinated action and similar amongst the Chinese diaspora - actively so, beyond the passive effect of censored content.
None of these things are to say that billions of people can't happily use those platforms, every day, all over the world! But they also cannot be completely ignored...
And before anyone starts with "but the American government is just the same!", it's not, it's really not. The Chinese government is a literal totalitarian, authoritarian, one-man dictatorship. However much Trump cosplays as a dictator, and however much authoritarianism he demonstrates, there are huge qualitative differences.