The efficiency of the roman weapons and armour did depend to a good part on the roman military training, which was largely lost. The tower shield is dramatically effective if - and only if - used in a closed formation that is trained to stand together. It is too heavy to be used actively, so it works great if your shield covers your left side and the right side of your neighbor. In the less trained and organised formations of the middle ages, that simply didn't work. Fighting together in the extremely close quarters that the romans did required extensive training.
