I don't know about Unicom as I use China Mobile.

When traveling outside of China on China Mobile, they have a plan where maximum data charges are capped at RMB 30 per day, with no data limit. You can't go on Google or any other blocked sites in China even if you're overseas, but this is the same with Unicom or any other China-based telco.

RMB30 or $5 can still get expensive, but this is better than the nightmare stories I've read about US telecom subscribers getting charged thousands of dollars for overseas roaming.