Yes, I applaud the efforts of any new companies coming into aviation attempting to bring new engines into the marketplace. As Kurt says, nothing will ever change unless some new things are tried and eventually proven.
The truth though in aviation is that nothing is sound until well flight proven to be so. Even the new engines from GE, RR, Pratt and CFM powering new twin jet airliners are not fully approved for all over water routes until substantial actual flight time and reliability history is accumulated. Even then, stuff does happen as seen on the RR powered A 380s. Nothing is perfect, despite hundreds of millions of dollars of R&D.
The Thielert was not especially prone to catastrophic failure but rather, slow, well premature ring/ piston death and frequent teardowns of the clutch for inspections. This meant the overall operating cost per flight hour were well beyond projections and the downtime was added to that cost, especially for flight schools. Lawsuits in Europe flew and disgruntled operators either dumped the aircraft or waited out a better idea, hopefully in the form of the new Austro engine. Operators had to use them or sell them so yes, they accumulated a decent amount of flight hours but simply did not even come close to delivering on lower operating costs or TBR.
The DA42 here was apart more than it was together after many problems, SBs and inspections. I saw the clutch apart on the Thielert once and it was just a stupid design IMO. These engines simply were not viable as delivered with removals taking place at 200-500 hours typically due to excessive oil consumption.
No doubt Thielert has learned and improved on the original design however the DA42 is no longer being produced with their engines. SMA under new ownership is also no doubt applying fixes to bring engine life up to the expected levels. At the same time these things are happening, the costs for the manufacturer and clients goes up through warranty claims, new engineering costs and maintenance costs, making people wonder if the high initial and operating costs were worth it compared to the fuel savings. With the new costs having to be eventually passed on to the market to ensure corporate survival, the overall economic premise is put into question. Indeed, the fiscal problems at Thielert were, in large part, due to an unsustainable model partly caused by what was an insufficiently developed design. I don't think you can call their attempt successful. The good sales of diesel Diamonds was based a lot more on marketing hype than real world overall operating savings.
What I'm trying to say with all of this is that DH will also have to prove that initial costs, operating costs and reliability are at least as good as the crusty old Lycoming on a per flight hour basis. Lower fuel flows are meaningless in the big picture if the engine costs 50% or 100% more to acquire and has to be replaced prematurely. DH has had many years now (!) to complete exhaustive testing and redesign prior to market release so expectations will be very high for this expensive engine.
One does start to wonder how viable their financial model is though after all these years of engineering and development and the substantial costs involved. How many engines do they have to sell and at what price to just get back to even now? In reality, often these costs sink a new aero engine project almost before the first engine is delivered. The reality being that overall, the project must show a profit to stay alive.
A sound mechanical design coupled with a sustainable and realistic financial plan is the only way any new engine design, either SI or CI, will be with us for as long as the Lycoming. I truly wish DH, Austro and SMA the best of luck here. In many parts of the world, good diesel designs are really needed and who wouldn't want truly lower operating costs and fuel flows? The problem is, nobody has done this yet with an aero diesel so I suspect it is not as easy as it seems. SMA almost certainly never turned a real profit on the low number of engines delivered and having to fix so many under warranty. When someone can offer a $20-$25,000 180-200 hp diesel which really does go consistently 2000+ hours before overhaul, without costly inspections and maintenance, while burning 25% less fuel than a Lycoming, then the day will have truly arrived. As I have said before, the very high gas loadings, vibration characteristics and high continuous power settings in aviation use make the design of a competitively priced, reliable, long lived, lightweight aero diesel very challenging to designers and manufacturers. If it was easy, someone would have done it decades ago because the diesel has been around for a very long time now.