Isnt a problem if China has 68% of the hashrate?
Couldnt China do a 51% percent attack?
I will point you to an old thread,
Is CHINA CONTROLLING Bitcoin?.
Are you talking about China, their government performing a 51% attack? Or some mining pools?
You just have to understand the money and resources involved pulling it off and what incentives the bad actors doing a 51% attack, it's not worth a try.