I do diesel engine surveys. Have done them for 20+ years.
A few tidbits:
For insurance and financing, an engine survey is generally not required. Just the hull survey. This can vary depending on the boat and the parties involved. Usually they just want the hull survey report, my report is for your use only.
I usually work together with the hull surveyor. We work as a team and share findings, usually on the same day.
Some hull surveyors "do the engines" too, and that is fine, but usually do not go into the depth that I or another mechanic will go.
Each engine guy (including myself) have engines that they feel qualified on. This is an area that is not really regulated. As far as I know, there really is not an organization that manages "qualifications". I hold none. But I will refuse a job if it is an engine I don't know, and there is someone else I think could do it better. For example, I will not do MAN, MTU, high HP Volvo and high HP Cats. Am experienced with smaller Cats, Cummins and Volvo, also Ford, Detroit, Deere, Yanmar, etc. Lots of what I look for is not brand specific, so I can be helpful even if I don't know the specific engine very well.
A mechanic that is certified in a certain brand knows troubleshooting and things like rebuilding the engine, but that does not mean he really knows how to analyze a marine installation. I have heard of a certified mechanic doing an engine survey where he stayed in the salon with a laptop plugged in, taking a lot of data (which is good) but barely went in the engine room and missed some water and exhaust leaks, and missed a broken engine mount.
Some hull surveyors have a lot of engine experience and can do a good job on both. The good ones know their limitations and will express that to the customer, and will have a list of engine guys that they will call on to assist if they feel out of their "comfort zone". That's when I get a call.
To the buyer, much comes from doing a phone interview with the hull surveyor. If a relatively simple engine, his inspection can be adequate. For a turbo-aftercooled engine, if he does not measure blowby flow, boost pressure, charge air cooler and other cooler delta-T's, etc, you might want an engine guy in the hole.