Well, to be accurate, the issues with the MK6 TDI (with the 2.0L common-rail engine) are unique to the MK6, and were not inherited from the 1.8.
Part of the challenge came from the switch to Ultra-Low Sulfur Diesel (ULSD), which stripped much of the lubricating qualities from the fuel. The High Pressure Fuel Pump relies on the fuel for lubrication, but the quality of our diesel here in the USA is sketchy and inconsistent, so many of these cars are grenading their fuel pumps. For the first year or two, VW was blaming the customer of accidentally putting gas in the car (gas has no lubricating qualities at all), but eventually this was proven largely false. Instead of putting better fuel pumps in the car, VW continues to simply replace them whenever they fail, along with the fuel injectors, fuel lines, fuel tank, and so on, to the tune of an $8k job. Why so many parts? Because when the fuel pump fails, it sends shards of metal throughout the fuel system, ruining everything.
Many MK6 TDI customers are also complaining of intercooler freezing problems. If you drive the car in the right (or wrong?) conditions (temps around freezing; high humidity; a gibbous moon; etc.), ice forms in the intercooler. Then, if the temps rise while the car is parked (outdoor temps rise; you park in your heated garage; etc.), the ice melts into the bottom of the intercooler, rendering the car either difficult or impossible to start. If the engine hydrolocks, bent rods could result. In 2011, VW designed and released an intercooler kit which largely fixes cars that get towed to the dealership when they can't start. Has VW started building these improved intercoolers into the MK6 TDIs to avoid problems with new cars? No, the 2013 TDIs continue to roll off the assembly line with the crap intercooler, even though VW designed an improved part.
I'm not saying that the 1.8 TDIs didn't have problems. I'm just saying that the 1.8 TDIs didn't have these
It's so nice to get into my Impreza and not have to make the sign of the cross on my chest before turning the key.