1. No. Some blocks are acceptable. If someone gives you an uppercut, you can block with guan sao as it has dominant energy.Two thoughts that come to mind. 1, You block a punch 'bone to bone', so isn't blocking as a concept a bad idea?
2, You never see Wing Chun in UFC. So, if blocking with your hands is a bad idea, wouldn't that mean you just said that training in Wing Chun is a bad idea?
Just thoughts
A kick usually contains power at the end (the foot) and you can sometimes intercept by blocking the knee/thigh, which is what I thought you were doing. You have since stated that you use guan sao against the persons foot, where the full blast of the kick is. Any block where you are attempting to stop a larger force coming in with a smaller bone is a bad idea. Yet if you change either the force of the attack coming in or the size of the bone you are blocking with, blocks can be useful
2. Not really understanding your logic. Many UFC fighters have trained in wing chun and some good MMA fighters started out with wing chun.
The wing chun I train never blocks kicks with hands, and to be honest, any wing chun that teaches you that should not be trained
I appreciate SiJe's post which describes the stopping of kicks as deflections rather than blocks. But Cuong you have said that you use guan sao which is not really a deflection