opisska
rabid twitcher
China is mostly how difficult it is to access most of the good birding spots without a good understanding of Mandarin (only speak English and Spanish here), plus many parts of the country are limited on what tourists can and can't visit unless they are with a guide from China (kind of what happens for Cuba if you want to visit any of the protected areas). Plus not to mention how bad the driving would be.
As for Gabon and Guyana, it's a combination of difficult to coordinate on your own, country has limited connection, so it's best (and likely safer) to ask a local guide to create and itinerary for you and have them join you since the regions are mostly composed of lowland rainforest habitat with high levels of endemism/little known species. So it just seems like a better option overall, but then again, I'm sure there's a few birders out there that would still try it on their own like the "Budget Birders" couple, but definitely not countries I'd feel comfortable doing on my own (Papua New Guinea is also in that list but that one is usually seen as a tours=safe option).
In China there is no "driving would be bad" - you simply can't drive in China without a Chinese license. So that spares you this problem However we have an entire active topic on the forum about where some people argue from experience that independent birding is indeed possible.
I actually looked into Gabon and the problem seemed that there isn't unguided access to many places allowed. But in Guyana (and the two other countries to the east of it), free movement should be easily possible, it has been actually suggested to me that this is one of the best areas for independent rainforest exploration in the world.