Life Cycle Assessment of Multi-Megawatt Airborne Wind Energy | TU Delft Repositories about an available thesis, page 35 about the ground station using hydraulic technology:
The presented drum is only a shell, without an centre axle, inspired by hub-less wheels. It rotates on 4 off-center axles, each connected to two hydraulic motors. The 4 axles are supported by a heavy steel support structure. It effectively relocates the structural mass from the rotating component to a static component to reduce inertial losses. The shell is considered of CFRP material, with a mass of 1500 kg. It is connected to the 4 shafts through 1 or 2 gear ring(s) on the inside of the drum.
The reel-out (productive phase) tether speed is a fraction of the wind speed, conventionally being 1/3. This leads to a huge generator at low rpm or a generator with a large gearbox when the system scales up. Hydraulic technology could allow both continuous power in spite of alternating reel-in and reel-out phases, and scaling concern solving. Problems of leakage can occur, as mentioned in the conclusion of the paper below:
Hydraulic technology does not appear to be currently applied for large wind turbines where the weight reduction of the nacelle is however desired. Thus conclusion mentions also: “Secondly, the development status of hydraulic components is limited.” Perhaps the costs can be a concern as a complete installation is a bit complex. I maybe see some other possibilities.