The problem is that Google’s server aren’t accessible in China which means wifi is completely broken unless you use a VPN. Even when not connected over wifi, you have to live with a permanent exclamation mark by your data connection indicator telling you that the phone can’t connect to Googles servers.
This isn’t that big of a problem is it? The exclamation mark is just a slight visual annoyance that you would quickly get used to. But functionally, everything works as should in China.
The biggest annoyance would be if you first turned the phone on in China. You wouldn’t be able to get past the setup wizard without being able to connect to Google and you can’t even setup a VPN at that stage to connect. That’s the absolute worst headache.