The presence of a credit system to begin with, with puts people in the mercy of corporation (private) or the bureaucracy (public neither in China nor in most of the world, perhaps with the exception of Switzerland, means "directly controlled by the people").
Public services, especially those run by China, don't exactly have a stellar record of being uncorrupt and accountable. What if the system is abused to shut down a competitor on the surreptitious "donation" of another company? What if being LGBT or the wrong religion is made demerit-worthy?
It would bother me a lot less if it were private and governed private services only. There's more accountability in the sense that, there will always be services available that don't require the credit system, and there will be multiple competing bureaus ala the current credit system.
The private system, even as bad as it is now, has to reasonably correlate with credit risk to some degree, otherwise companies wouldn't use it (or would use the competitor). A single, gov-enforced system has no limits like that. If they want to give you fewer points because you're not aligned enough with the party line, you get fewer points, and you're more marginalised now. There are no big customers to say "don't want to use this, it doesn't provide me any benefits", because those companies want to be able to continue operating. (Before anyone mentions it: Yes, I know that at the moment only well defined and documented cases are counted. It's not like China would change the rules to, for example, lower the score for liking Falun Gong or other ideas, right?...)