I can't comment on other Disco's as I haven't owned them. But my D2a TD5 ownership has been a lot more trouble free than general opinion seems to expect of a vehicle loaded with electronics and an electronically controlled engine and transmission. I have had some issues with it yes, but thankfully so far it has not left me stranded.
First 4WD trip - hole in the fuel line - finished the track and drove home no issues.
More recently, rubbed the transmission wiring loom through exposing the copper of some wires on Day 1 of a 4WD trip 200kms from home. The vehicle crawled along for a day and a half off road and got me home to repair.
Most recently, worn alternator, after 280,000km wasn't keeping up with charging the battery. Luckily it was still outputting enough that I managed to drive to and from work every day for a fortnight before being able to replace it.
I've also had the fuel line leak at the back of the vehicle above the fuel tank on the chassis rail and the FPR leak twice. Generally there's always something trying to leak engine oil and or coolant out - currently I suspect that the o-ring between the block and the accessories/water pump bracket needs to be replaced again.