As a scuba diver, I can attest to the effectiveness/ineffectiveness of neoprene. The most effective way neoprene works is to allow a small amount of water to enter, the body warms that water, the glove limits water exchange, and the overall system is very effective in storing heat. If water does not enter the glove (or wetsuit), that region of the body will be cold. If too much water moves in and out, that part will also be cold.
The same glove in 40F temps will be ineffective if dry, but very effective if wet inside with no water exchange. Ideally, a small amount of water enters, the body warms up that water, and everything stays warm.
My most effective cycling mitten is a Pearl Izumi PRO. It is, however, a sensitive mistress. Above 28F, my hands sweat uncomfortably. Down to 18F, my hands are just right. Below that, I need supplementation, e.g. pogies. YMMV.