Notched ones are fine for measuring the board itself but in the end you need both notched and not. In the case of an untrue fretboard, it might not be level throughout and high spots on the wood might be present on one side of the board and not the other. In the end, the frets are what matters and the board is somewhat secondary.
Here's an example of a plek analysis along the e string path (the grey at the bottom is the fretboard.) although the graph exaggerates the fact that there is a 0.25mm variance, it still exists. Unless you are re-fretting and sanding the board, it doesn't really matter. The frets are what matter.
I can't speak to the accuracy of the Nomad 3 sided guitar straight edges as I don't own one but the idea is cool.