Are all the shifting "complaints" due to the Campagnolo style shifting vs. Shimano? Or is the thumb trigger really hard to use?
I think more the former rather than the latter. As far as Campag-style thumb shifting goes, the ergonomics are about as well-designed as they can be. Some users have reported that the mechanism binds a bit and is hard to press, but that's not a problem I've experienced.
My explanation is that thumb shifters will always have these problems because you only have 1 thumb wrapping around the shifter, (versus 4 fingers on the other side), meaning that thumb shifters will always require the user to move their thumb and loosen or change their grip to shift, while paddles near the brake lever let you maintain grip with your remaining fingers. Furthermore, shift paddles near the brake lever are reached by extending the finger rather than contracting it, while LTWOO's thumb shifters are positioned
behind the thumb so they can be reached from the drops, meaning the thumb would have to be contracted to reach it. But, there is no way to contract your thumb to reach the button from the hoods, because the thumb is already contracted to grip the hood. This means you have to move your hand.
Maybe there is a mythical thumb shifter design that avoids this problem. I've thought of something like two small thumb levers connected by a bracket under the hood body. One near the brake lever pivot (like on the old Shimano Sora) so that your thumb can extend to shift from the hoods, the second near where the current lever is, but with a smaller profile designed to be exclusively operated from the drops. One can dream.