I agree that Tung Ping Chau is underwatched. The same is also true of a number of other sites in Hong Kong (see for example SL TAi's recent posts about Leadmine Pass). I think this is largely the result of birders in HK tending to visit a site that has a reputation - Po Toi is one such site, and gets a huge amount of observer coverage (Thanks Geoff!)
I try to visit TPC at least once per season (I have often posted sightings on the forum). Living in the northern New Territories, it's actually much easier for me to access than Po Toi. On some days it can be very frustrating, especially because you arrive late in the day. On other days it can be packed with birds - this is especially true in winter when the shrubland on TPC is full of thrushes. As Mike has mentioned, one problem is that there is a lot more potential habitat for birds to disperse into than on Po Toi, and a lot of this is not accessible. The best areas seem to be inland, around the police station in the north of the island and behind the reservoir in the south of the island.