CH. Part of the problem is that the specialists don't know either.
And, there is one sentence that was told to me years ago by a pal who had been ribbing for some time. It rang true with me also.
"They'll tell you anything."
When I was investigating my diesel engine choice, I spoke, at length, to those who are purported to know. You have to read between the lines. I was fortunate because there was a series of boat shows over a weekend on the west coast and both the Barrus reps and the Volvo agent were there. Also, just by chance, I met a friend there who had a KAD 44 rig which he had just sold and he was going for a Yanmar and 3x. We spoke to the Barrus guys at length but we had to make sure we asked exactly the right questions to dig out the truth. Watching their body language and there glances to each other told as much of the story as their words. This was so much the case, the pal was about to cancel the Yanmar. He is completely non technical and, from his recounting his experience, it seemed obvious that his Volvo problems/disappointment stemmed from poor maintenance by the local agent rather that a weakness of the Volvo. We got the truth from the Barrus guys eventually but very reluctantly. This was the deciding factor and we've discussed it before on this forum.
Finally, for me, the choice lay between twin Mercruisers or a single Volvo. Now, that took a lot of deliberation. There were so many reasons to go with each. One reason for the consideration of the twins, which is relevant to this thread, is the use of the Alfa drive. Very capable of accepting the 120hp, relatively cheap, available in various ratios, contra rotating at no extra cost, a huge range of props, some very cheap, £100 or less. Remember, our Volvo duoprops are costing about £1000 a set. Silly money really. Quite why I finally opted for the Volvo I'm still not entirely sure! I think the persuading factor was the extra speed I would achieve. I have very much enjoyed my present boat and it will cruise all day at 45mph. I wanted to continue to do the same.
However, back to the thread. I've other friends who have used the 3x extensively. Lots of them!(3x's, that is). Yanmar cracked blocks too. They made a comment to me that I've not forgotten. They said they would really like to be rid of the feeling that, one day, they may have to call for a tow. It always prayed on them while they were out on the water. And, if they were buying again, they would be willing to consider reduced power to gain reliability. For me, the choice of the DPX was partly to do with predicted speed but two other factors were also important. One was the wide gearcase which is a proper aerofoil in section. This will give very good low speed steering. The other was the hydraulic steering using external rams. There is one thing I learned early on. On a fast boat (and on any boat to some extent) tight, responsive steering is worth its weight in gold.
There's lots more to say but I'm blethering so I'll stop here (for now
However, when buying any marine equipment, remember, 'They'll tell you anything.'