I'd love to see this come off and am confident it will - one day! I have been saying that since the 80's though, when Zoche diesel radials were going to be the next big thing in aviation. There have been several since then which have promised a lot but struggled in delivering, ask anyone that had a new Cessna grounded due to clutch issues with the Thielert, this even before financial woes caught up with them.
Again, on the longevity and cost issues - Granpa's Ferguson is a far cry from a modern high revving, common rail, turbo diesel. They do break, and damned expensively at that, anyone here had to pay for a rebuild on their turbodiesel ute, never mind European luxury model? Enough to make your eyes water and often runs a replacement engine a close race, at $17-20,000! Some are better than others, but I have seen em all in bits accompanied by much wailing and gnashing of molars.
Someone also raised the maintenance issue, well manufacturers blurbs notwithstanding, most mechanics are recommending shorter oil change intervals and experience in cars suggests that higher maintenance costs take many, many km to set off against lower fuel costs per km for diesels.
So again, I'd love to see it and am sure that one day, with a lot of money, someone will get it right, but I don't know that I'm ready to join the test pilot team just yet.