There are two problems with post-2009 TDI engines:
(1) In 2009, VW/Audi introduced a soot filter to meet emissions requirements for particulates. The screen is immediately after the last cylinder's exhaust valve on the exhaust manifold. The screen accumulates soot particles, and in order to clean it, VW and Audi use the tactic of burning off the collected soot periodically (like a self-cleaning oven) by using a periodic cleaning cycle that mechanically holds the exhaust valve on the last cylinder open longer, increasing the temperature and free oxygen exposure of the soot screen and burning off the soot. The problem: post-2009 TDI engines have been turning up with burnt exhaust valves on the last cylinder - big $$$ valve job if out-of-warranty.
(2) The turbo unit on TDI's have a long history of carbon build-up requiring replacement at 70,000 miles or so - depending on driving habits (highway miles are less problematic than frequent short trippers). HOWEVER, to improve fuel mileage VW/Audi leaned ~OUTT the diesel injection which results in a hotter burn (see above and also increased NOx formation - which is the pollution control problem). Beneficially, this reduced the carbon buildup on the turbo unit. HOWEVER, if you get the "software fix" to reduce the NOx emissions (like they do on the test cycle for emissions testing only), what it does is increase the injection fuel volume for a cooler burn - that produces more carbon and soot in the exhaust - both clogging the soot filter and increasing carbon buildup on the turbo unit - shortening its life. It also reduces fuel economy.
So, if it wasn't for (1) - and a cooler burn will help somewhat, a decent strategy would be to buy a used TDI from 2009-2015 and NOTT get the "fix" to the emissions control. Register the vehicle in a state with no smog testing (there are many). And run it as it is - good mileage and you'll be able to buy it at fire sale prices.
Butt sadly, (1) applies. So that frustrates the purpose.
Once folks have the VW "fix", they are going to hate both their mileage and how frequently they have to replace their carboned-up turbo units.
The pre-2009 TDI engines are basically great (with the turbo proviso). Those things will last for 500,000 miles or more (nott counting turbo replacements).
Americans and Euros want three things, butt engineers are limited by the laws of physics and chemistry and can only deliver any two of those three things.
(1) great fuel mileage
(2) very, very low emissions
(3) affordable to the average consumer
Pick any two and the engineers can give them to you. You can never gett all three in one pot from a diesel. Gas engine yes, diesel no. This is why Honda, Toyota, and Nissan (which had diesels in the '70s and '80s in pickups) don't offer diesel engines in US vehicles.
