I was in China a couple months ago and ExpressVPN worked fine for me, are they actually using their own custom protocol?
ExpressVPN is the only one afaik that is allowed by the Chinese government. You can assume that it is actually not "private".
When I was in China last April, SSTP worked fine (though long-lived connections tend to become slower over time, and then need a few minutes of cool down before being usable again). Most Chinese people I met were using shadowsocks.
Many friends indicated Express was down almost an entire week surrounding the big meetings. Not as reliable as it once was. Or so they say
Doesn't matter if it's not "private" if you have certified encryption though, right?
I think his assertion is that as it is "allowed" by China's government that they somehow have access to the keys (or the VPN provider is keeping a buffer of the traffic and has an arrangement to provide it to relevant government agencies upon request).
I have no experience with ExpressVPN, so I can't help you with that.
But in a vlog I've watched on Youtube, the host of that vlog said "Over the last coupe of days, ALL the VPN is been very difficult to use". So, I guess that includes ExpressVPN.
Here is the video if you interested: https://www.youtube.com/watch?v=EuEdYvQmVFg (5:20)
Im in china at the moment using expressvpn (been using it for a year by now) and since about two weeks only three server locations work well (Hong Kong, Tokyo, Los Angeles). Some others work off an on. Before that most locations worked and some of them, Taiwan for example, used to be very fast. Its still usable for streaming and surfing but I'm afraid the end is near. I think sometime in the future one will have to go with shadow socks and or similar protocols/solutions but until then expressvpn is quite convenient (mobile client, router with expressvpn client).
In China, you may need to use one (Or multiple) of following:
https://github.com/shadowsocks https://github.com/v2ray https://github.com/XX-net https://github.com/ginuerzh/gost
And +https://github.com/gfwlist/gfwlist for automatic proxy switch.
Those applications may require a dedicated server or VPS to run. Once you set it up, it will act like a relay between you and the host you want to access (So that server or VPS must located outside GFW's shadow. And you better set it up and get it well tested before you move to China).
If you don't want to setup a server all by yourself, you can use Lantern or Psiphon, but they are considered not safe as you don't have any control once data leaves your machine.
I personally use Shadowsocks + my own one made with Golang. Both of them works very good for me. Some people may had problem with Shadowsocks but cause of those problems remain a myth.