Actually, the amount of "evidence" depends on how you look at it.
For example, as I'm sure most of you know, the evidence linking coffee consumption with lowered risk of diabetes is fairly well established at this point. It is currently believed that this is due to the Chlorogenic acids present in coffee.
Two Tradional Chinese Medicines which are also used as anti-diabetic medicines (and for weight loss, blood pressure and lowering cholesterol, FYI): honeysuckle (jinyinghua) and bitter (nail) tea (kudi cha). Both of these are also exceptionally high in Chlorogenic acids.
Is there a connection? Almost certainly! Has it been properly researched in the West? No!
But, if they did a long term, controled clinical trial, I would not be in the least surprised if they found that it reduces the risk of diabetes.
Furthermore, there are other traditional chinese medicines with excellent safety profiles which have very strong theoretical basis for being anti-diabetics. For example, DNJ (1-deoxynojirimycin) found in Mulberry leaf has similar activity as acarbose, which a perscription anti-diabetic drug. They both work by inhibiting hydrolysis of certain multi-unit sugars to allow absorption. This slows the absorption of sugar, so it lowers peak glucose levels.
To my knowledge, there is already solid evidence that high peak glucose levels are bad for you. So, perhaps more research needs to be done with DNJ, right?
Furthermore, there is ongoing research that I am aware of regarding Bitter Melon, which will hopefully provide convincing evidence about its effectiveness as an anti-diabetic.
These are exciting times, and I really looking forward to the next few years. Herbal treatments for diabetes are about to enter a golden age!