Any difference in braking between max manual and max auto on a wet runway is relatively small. The practical point is more about how the capability - deceleration as per Boeing diagrams - is used to minimise distance.
Any delay in applying manual braking has to be balanced against autobrake's advantaged of quick and consistent application. Boeing highlights this in their technical bulletin, where in-service manual brake application may take 4 - 5 secs vs the 1 sec assumed in the data, with an increased distance required of up to 1000ft.
A greater difference, and safety hazard, is an inappropriate choice of autobrake setting and/or misjudgement of the runway conditions, which may require the use max manual brake late in the landing.
Another issue with autobrake is the human 'automatic' functioning which may reduce emphasis on the need to assess runway conditions, to consider distance additives greater than the minimum 15%, and use a higher autobrake setting.