Can be a bitch on the sellers behalf sometimes. I remember I had with an old car, a younger guy bought it "subject to satisfactory mechanical inspection", and paid a deposit.
Now this guy went and checked out the car for the inspection. The buyer ended up coming back saying "my family mechanic *said* it was exhibiting signs of worn headgasket and abnormal wear". But I was pretty dubious about that, and had no way to contact that mechanic to find out exactly what he did said. He could have actaully said "yeah, good car, but the headgasket is slightly down, but should be fine like this for a quite a while to come".
Sure it was an old car, but I had been driving it for years and long distances, in hot temperatures, etc and it was exhibiting to me absolutely no signs that the car had such problems. So in the end, I suspected either the mechanic was assessing at being in pristine factory condition (e.g. for a car under 5 yeares old) and not assessing it commiserate with age or price, or the potential buyer was distorting things to get out of the purchase.
In the end I had a strong feeling he had changed his mind on the car (as young guys do), and was looking for any excuse to get out of it. Ended up giving back his deposit (which i wasn't too happy about as I was still didn't suspect it had any mechanical probs at all), but a lot of being stuffed around. In the end another guy bought it about a month later. He bought it on the spot and I assume had no problems since - he just knew it was an older car and not going to be perfect for a car 15 years old.
Any car I think if you look hard enough you can find anything on a car.
My mate that bought a car over the weekend (which was an R33 actually), just took it for a drive, gave it some stick, let it idle for a bit, do a few full lock turns and that was basically it. Car drove fine and very smooth, made no funny noises, looked genuine, seller seemed very genuine and that was enough in his eyes, and mine. Will find out in the next few months, but i very much doubt anything is going to show up.
My car I bought privately.. took it up past 200km/hr did all the above, and again that was enough and never bothered with a mechanical inspection. Never had one mechanical problem in 20 months or so of ownership. If you push a car to its limits if it doesn't fail then in my eyes that is good enough Often i think mechanical inspections can be a wank, as afterall you are buying a 2nd hand car, and to be honest most mechanics are still grounded in the past pre-EFI and ECU driven cars, and I think a lot of users on here would know more about skylines and imports in general than your local garage mechanic these days.
I guess one of the things that *is* important, if not most important to assess the overall health of the engine, is getting a compression check done. If the engine is losing compression in one or more cylinders basically means the engine has to come out and be rebuilt, and its also losing power. Unfortunately that is a bitch of a bitch on the RBxxDE engines as its a 20min job just to get to the spark plugs themselves, then measure each cylinder takes a while. I don't even think most mechanical inspections go to this level these days??
If you can't tell there is a problem from driving it properly or giving it the once over, its not a major problem. If there is, you don't buy it. Simple.