One of the reasons, as you stated, is there are now 3 PCUs powering the rudder. It is, therefore, possible for you to apply excessive mechanical force to the rudder which could result in structural damage. There is also no pressure reducer on the standby system as there is on the main systems and this will allow full system pressure to be available above 135 knots which will only be a problem if you're ham footed or stupid.
Or it may indicate a pressure differential between the 2 main PCUs which might suggest a jam. This is less likely and you still have enough rudder control with the standby system so it's not considered a problem.
All the NNC is trying to get you to do is be gentle with your feet. Landing in a crosswind does not require full rudder, don't bloody kick anything and nothing will break off.