Volvo's 12000 mi oil change interval was crap, and the cooling system is beyond retarded. The D24 is not common, and not tolerant of improper service. The TDI isn't that tolerant either, but being common, there are a good amount of folk who know how to fix them. If you don't do the work youerself, a TDI can easily cost more than the price of the car in repairs.
If you do the timing belt right, get the crank pulley bolted on tight, and don't overheat, neither one is prone to sudden engine failure. fail at any one of those tasks, and it will be a major job or engine replacement.
Half the dead D24's Ive seen were killed by somebody not torquing the crank bolt enough.
^^ 100% agree with all of the above. The other half were either killed quickly by overheating, or killed slowly by incorrect (non-synthetic) oil use or by owners who slowly got more and more tired of their poorly-starting, poorly-running, loud, smoky Volvo Diesel after some gorilla mechanic changed the timing belt w/o proper tools and methods, didn't get the cam and IP timing set right, and it never ran the same again.
Provided proper basic care and maintenance, a D24T is as hard to kill as a redblock, or harder.
More TDI's on the road because there number produced and sold in the US is greater by orders of magnitude, and the cars are at least 10 years newer, parts and special tools are more available, and more mechanics have some clue what they are doing working on them. There are more late-model gas Jettas on the road than early 1980s gas Volvos, too.
TDI's can be disasters too if not worked on right, but one reason they are a little more difficult to really screw up than a D24/T or other old-school mechanical diesel is that the computerized control of injection timing gives the engine some degree of ability to compensate for the work of a sloppy mechanic. If a guy does a timing belt on a TDI without the proper tools and procedures and just slips a new belt on like you would on an Accord, without doing the work to set cam and IP timing, the TDI's ECU can move the injection timing around enough that even if it is pretty far off, the car will still start and run basically OK. It will not run the way it should but in most cases it will run well enough to not turn the car into a complete turd, and many owners will not really know the difference.
By comparison, if you did the same thing to a Volvo diesel, or an old 1.6L Rabbit diesel, or AAZ, etc, then if the timing is off it is off and nothing will be there to compensate for it. This causes the motor to run like trash, and then maybe one or two glow plugs dies, the car starts like dog crap and runs like dog crap and makes the owner feel like dog crap, and off to the junkyard it goes. And even though it is all due to a mechanic's negligence and nothing to do with the car, the engine gets blamed, its bad reputation grows, and before too long the whole thing becomes a self-fulfilling prophesy.
Two diesel Volvos I regularly service are just about to turn over 300,000 miles, on original motors. Neither one has been babied and both are strong and healthy. Proper care and maintenance.... like anything else.