The hand held tachs are generically called photo tachs. They are very accurate because they use a crystal based digital reference to measure speed at the flywheel with a piece of tape that they detect light pulses that is bounced of if it.
It probably isn't critical to set the wot rpms of the old Lehmans and Perkins that many older trawlers use. But it is very important to get the maximum wot rpm right for modern, high output diesels.
The Faria tachs on my 2006 Mainship 34T with a Yanmar 370 engine were off 200 rpm. What looked like 3,600 rpm was really 3,400 rpm. 3,400 rpm was fine as the engine is rated to produce 370 hp at 3,300 rpm, but it might not have been ok and I could have overloaded the engine with too heavy a prop if I had relied on the Faria tachs and reworked the props to reach an erroneous 3,400 rpm.
So check your wot rpm with a photo tach, especially on high output diesels.
David