What are those reasons?

The Chinese government cannot be trusted. PRC-based companies are required to cooperate with the state intelligence apparatus. Not that much different in the US and the US-aligned world, but at least there is more transparency, open discussion, and defiance.

All the reasons (privacy/spying, government unaccountability, etc.) that would make basing a tech company in the US bad are worse in China.

It does seem, though, many are (seemingly in bad faith) increasingly conflating criticism and scepticism of the Chinese government with racial or ethnic animosity. It is almost identical to how discussions surrounding the Israeli government are conflated (seemingly in bad faith) with anti-Semitism.

E: spelling and placement

but is Opera operated by the Chinese Government? looks like it is from what you are saying but dang I have no clue.

They don't have to be fully operated by a govt in order to exert operational control in certain areas of interest.

We have seen this with Zoom: China-Based Executive at U.S. Telecommunications Company Charged with Disrupting Video Meetings Commemorating Tiananmen Square Massacre

https://www.justice.gov/opa/pr/china-based-executive-us-tele...

Uyghur genocide, the Dalai Lama, Tibet, Hong Kong, Inner Mongolia, West Philippine Sea, Spratly Islands, Taiwan, Galwan Valley, Mekong River, Nepal, Laos, social credit, pervasive surveillance, Falun Gong persecution and organ harvesting (debated?), stealing tech, economic protectionism, ...

America does bad too, but we can't act like this state is any better. Some of the things they're doing to play catch up are appalling.