I have already outlined above one of the main issues with these systems from a vehicle development perspective.
The flip side of that is that the systems are nigh-on bulletproof, often providing two decades or more service with no maintenance at all. The bearings on the suspension arms ususally give up before anything else and the units are good for at least two bearing sets, which last about 15 years...
The only vulnerability is the interlinking pipes beneath the car. Corrosion here will cause the system to lose fluid and the suspension to drop. An obvious improvement, but one never made, would be to use stainless steel pipes.
In my opinion, the earlier Hydralastic system was better than Hydragas. The "float and sit" ride produced by the former is unequalled in mass production vehicles and offers Rolls-Royce ride quality coupled with zero body roll on cornering. Hydragas does have the advantage that it removes the choppyness encountered if Hydralastic is used on smaller, lighter cars.
The achilles' heel of both systems is dive on braking and squat on acceleration, which is a product of the interlinking front / rear. It's something you get used to and nowhere near as puke-inducing as Citroen's Hydropneumatic system though.....
 Suspension is almost always a compromise. Softer suspension improves ride, but at the expense of roll and poorer handling. Moulton's systems provide the ride quality without the handling compromises.