It appears to be a spring scale, similar to those built into some tongue jacks used in Europe. If you don't mind shimming it up with blocks to the right height, and don't need more accuracy (I don't know how accurate it is) or more precise (it has 10 kg / 22 lb reading increments), it looks like a reasonable thing to use.
What is the problem with having a greater than the recommended 15% tongue weight, assuming the tow vehicle has the capacity?
As far as the trailer is concerned, the only potential problem is structural - you don't want to damage the tongue with more force than it was designed to take.
I agree that the tow vehicle hitch weight capacity is important.