I owned a 2006 Mainship 34T with that engine and now have a Pilot 34 with the same engine. I do all of my own maintenance, but am not a diesel mechanic by any means.
The real issue in so far as a specific boat installation is usually the exhaust system design. I have not heard of any issues with this on the 390.
The Yanmar 6LY 370 hp is a very solid engine.
Make sure that you use the approved Dexcool antifreeze. Regular, high silicate antifreeze can damage the aluminum exhaust manifold.
The raw water impeller is a chore to get to and most owners remove the pump to change the impeller but that job isn't easy either due to limited access.
The air cooler needs routine servicing, at least every five years and that includes disassembly, cleaning, new O-rings, lots of grease, reassembly and pressure testing.
Prop it to reach 3.400 rpm and cruise at 2,800 or less, maintain it right and you will have a happy engine that will last many thousands of hours.