In my mind, the novel problem here is that the Chinese government can use such information in order to socially police people that nominally aren't under their control. Google in Europe or America has no desire other than to collect info for commercial purposes, and there is no inherent desire to turn over private information to any government.
WeChat in America constitutes a potential extension of Chinese governmental control onto American citizens. This can be something as innocuous and reasonable as the denial of visas due to certain communications, or something worse. American citizens may get their Chinese friends and relatives in trouble for something they say. A few cases of these, and suddenly half the American WeChat user base knows they have to watch what they say unless they don't care about people they know. Moving out of the platform is not an option due to network effects.
China has long maintained that foreign tech services offer an opportunity for foreign countries to subvert their information controls, and has banned thousands of domains and companies under the guise of protection from foreign influence, commercial or gubernatorial. What's to say that China won't turn around and do the same thing to other countries?
To your point about no advertising BS in China, well you'd be right if it were 20 years ago. Unfortunately, advertising has become a booming industry in China. Take a walk in most significant Chinese cities, and you'll find the constant bombardment of information to be more garish than any American counterpart. The Chinese Internet is no different.