Witnesses are the single point of failure of the system. They essentially control the network and there are only 12 of them. You can imagine that if the rogue government (bankers or whoever) wants to take down the byteball system all they have to do is to take controll over 12 computers running witnesses nodes. This seems to be rather easy to do, especially at gunpoint. Moreover - this can be done without the rest of the network to even notice - if witnesses after being taken over by the rogue party are operated without interruption. Anybody who controls the 12 witnesses can do whatever he wants with the network - for example censor certain type of transactions. All of this is a contradiction to censor resistant trustless network that bitcoin is.