I doubt if the Disco 2 really has a lot of problems compared to the late Disco 1 for example. But one factor that may make it seem that way is that the D1 was in most respects a direct evolution of the original RR - and they had had twenty years to get it right in a lot of respects.
The D2 was very much a new design along the same lines, which means not only were there a lot of bits that were relatively untried, but the parts that were common (or at least interchangeable) often don't fit, so you have no choice but new, expensive parts specific to the D2, which makes repairs a lot more expensive than for the same repair on the D1. e.g. I understand wheel bearings are in this category, but the most obvious one is wheels - D1 can use at a pinch any Landrover wheel back to 1948 - but D2 has a new stud pattern.
While the newer design is not necessarily less reliable - and it should be more reliable - the need for parts specific to the D2 can make repairs a lot more expensive for a specific instance, even though overall running costs may be no more or even less.
John

