Language - whatever you may believe about the idea that "the world speaks English". It isn't true in China; China has the lowest functional English literacy rate of any nation on earth. There are two major languages used in the country - Mandarin and Cantonese. Mandarin is the most widely spoken, but Cantonese may be vital in the south of the country, particularly if your provider is located in Guangzhou (and to a much lesser extent Shenzhen). You cannot rely on the supplier's sales force (who are the only people likely to speak English in any given company) to act as an interpreter for you. If you can't speak the language you need to use a translation service to make sure you understand exactly what the situation is.
- Culture - Chinese culture is widely different to traditional Western business culture. The first problem you are likely to encounter is the concept of "saving face" which means that your factory may well be economic with the truth (not necessarily because they are being fraudulent) to save themselves public embarrassment and more likely to say "yes" to your demands when they mean "no". you will need a Chinese translate to English.
- Timing - when you visit a supplier as a prospective customer they are likely to be on their "best behaviour" (which sometimes doesn't mean very much) and all potential sources of embarrassment are likely to have been cleared away before you arrive (the ugly truth is that this could mean they lock all the child labourers away for the day for example) preventing you from making an accurate assessment as to their suitability.
- Understanding - it can be hard to validate even the basics in China, for example how will you know if that piece of paper purporting to be a business licence or for export permission is valid and genuine? The country is famous for copying anything that can be copied or faked and that includes paperwork.
- Information shortage - there is simply no real data held publicly in easy to access format that can help inform your decision, no customer review databases, no consumer protection society (or indeed much in the way of consumer protection regulation either) or reliable trade bodies to assist in informing your choice.
These barriers are not insurmountable and with care and planning it is perfectly possible to conduct your own inspection of your chosen factory. However the simple truth is that this is often an expensive and time consuming option, in today's commercial climate delaying your outsourcing decisions can cost you market share and revenue.