My understanding is that many believe that the RAT won't be able to provide enough power for braking as the aircraft slows upon landing (note: I haven't seen this assumption confirmed or denied by Boeing).
Whatever 'many believe', the Boeing manuals tells us this:
The main battery provides power for:
• airplane power-up
• APU start (assists APU battery)
• refueling operations
• towing operations
• electric braking (as backup power source)
• captain’s flight instruments (energizes essential instruments until RAT deployment)
In-Air Rat Only mode is active if loss of all electrical power to captain’s and first
officer’s flight instruments occurs in flight. The RAT energizes the captain’s flight
instruments and other essential equipment for flight controls, navigation, and
communication. The main battery provides standby power until RAT deployment.
There is no mention, in the entire manual, of the RAT having any effect on the wheelbrakes.