Why is Hong Kong's China extradition plan so controversial?

Exclude mainland China from the extradition law to provide assurances until China is fully democratic, recognizes freedom of speech, freedom to travel, freedom of religion, allows individuals to sue the govt and govt officials, etc ... and has a multi-party election system.

Have non-treaty extraditions go through some extra steps, like a random drawing of voters in the SAR who would have the final word on those other location extraditions.

Fears center on getting tangled up in China's opaque and politicised courts 

To say the least of it. People disappear in China’s legal system. And I don’t mean that figuratively. Communism is just authoritarianism by another name.

The biggest irony is that the name of the country is The People’s Republic of China, when in fact the only people who have any rights are the people in Xi’s entourage.

Hong Kong best free itself of Beijing and keep the pressure on

China will get its way. Sad but I just don't see any other way.