It's not really very controversial. If you need your car, and you can't afford to buy a new engine if it breaks, as far as I can tell you'd be certainly well advised not run a modern diesel car without a warranty. I don't know anybody who's owned one who hasn't had various funny problems with it, requiring regular trips to the garage followed by shrugging shoulders all round and a succession of replacement of random electronic parts.
To meet the increasingly stringent emissions controls, and people's desire for a car that is reasonably cheap and accelerates at a decent rate, the cars seem to have become rather complicated, and reliability has been affected.
Common points of failure seem to be:
- dual mass flywheel (presumably the regular swift ramping-up of torque doesn't help)
- turbo charger (obviously spins very quickly and unlike a petrol engine it comes on from about 1500rpm so it's used a lot)
- diesel injectors (high precision components operating under pressure)
- diesel particulate filter (can clog and require expensive replacement if car is not regularly driven hard)
- random engine/ECU "thing" that nobody can figure out (who knows what the root cause of this might be)
You might have better luck with a pre-common rail diesel car, but the fuel economy is unlikely to be very good and the performance - at least by modern standards - looks to be terrible.