I can't speak for anywhere besides HK and the US for acceptance. I used the card at the Airport Express station in HK, and there was no problem with card acceptance. You're better off using a 0% FTF Visa or MC, however, as they'll get better exchange rates. While the UnionPay card doesn't have any FTF in theory, UnionPay does maintain a small spread on the exchange rate. You can see my post here with the HK transaction was about 0.27% above what it would be had I used a Visa card. You're still coming out way ahead compared to a transaction with forced DCC or for the convenience factor if a merchant in the Mainland only takes UnionPay.

The US will accept the card, but you have to let a fallback transaction happen as the US terminals I've encountered do not support PBOC, only EMV. I imagine as more places switch to turning on EMV, they might deny your transaction for attempting to swipe a chip card. I'm not an expert on UnionPay's acceptance rules, but I'd imagine you'd have less recourse than a Visa or MC transaction with respect to card acceptance.

I concur that e-mail is the best option. My only feedback so far is the lack of features on the website. For instance, an address change requires a call or mailing in the form with the statement; there's no facility to do it online. They only recently added the option of doing a payment from an external bank account. (There have been mixed results pushing payments from this and associated threads.) However, I haven't yet had any problems with their phone support. For instance, I needed to link the credit card with the checking account for online access, and the link happened exactly as described over the phone. Usually when one receives the message, "Let me contact the department responsible for that," and you get transferred around there is a less favorable outcome.

Overall, I think it's a solid card product for my intended use case and has worked as advertised.