All overdrives are weak because it needs a small gear driving a big gear. Automatic transmission OD is no different.
You realise that the same 2 planet assemblies are used for both under drive one to one and over drive right?
Planet assemblies consist of the sun gear planet gears and the ring gear.
Depending on how torque is routed each assembly has 6 ratios. 4 forward and 2 reverse.
The only thing that varies is which element of the planet set is static which is input and which is output.