There was a lot clever about the Citroen system, which I got to know quite well.The current Range Rover suspension uses air as a "spring" and can be tuned to allow variable rebound rates to feel sportier or softer on various road surfaces. It uses a fairly complicated feedback mechanism to control air pressure at a distribution block. It's inflated by a glorified bicycle tire pump.
The weakness of the system is the air bladders at the top of each strut. Rubber tough enough to hold pressure can be stiff and will develop fine fractures with age - leading to failures.
My own (dearly departed) A8L had a similar system and the VW parts bin controller was temperamental, prone to locking up at full height and unserviceable by local repair shops due to its model year and computing requirements.
The Citroën Hydropneumatic suspension was 50 years ahead of its time, using mineral oil as the incompressible fluid. Early versions suffered corrosion failure when standard brake fluid was substituted.
This system required the disruptive spherical reservoirs that held Nitrogen in half the chamber separated by a membrane from the mineral oil.
This dampened the system on uneven pavement.
Mercedes Benz used a similar design in their Self levelling system.
Later Citroen versions called "Hydractive" employed more complex computer controls but the earlier versions were mechanical wonders.
It was robust, supple and flexible. It was purely mechanical, with no electronic monitoring or control valves.
Poor dealer service support doomed the brand in North America, often due
One of the things I liked best was that the power brakes - rare in Europe at the time - had a 2 way proportional valve system. The valve for the front brakes was fed by pressure from the main system. The valve for the rear brakes was fed by the pressure in the rear self levelling circuit so the brake balance altered with rear load. Genius!