Just a data point, but my M25, which has 2500 hours on it and runs well, takes a lot of cranking to start, unless I use the glow plugs for at least 30 seconds. Starts right up after that.
This is the exact behavior we have. We took it to a supposedly reliable marine yard that specializes in this engine. Old Lyme Marina, Clinton CT. They treated us well but the project manager told me the mechanic would not do a wet test due to fear the engine would kick over and damage his gage. When I asked him how that was even remotely possible with all injectors removed he could not give me an answer.
I suspect that not every mechanic has the same experience and has a good day even at the best yards.