I notice that neither authors actually play BOTH the guitar and the violin. This is a crucial weakness in their argument that the violin is "so hard to play" as compared to the guitar. Here, I attempt to highlight the skills that are required to play at a high technical level for both violin and guitar- restricting comments to the genre of classical music as performed on either instrument. I personally perform at an intermediate classical level with both these wonderful instruments. With the violin- as noted by the authors, bow technique is paramount. Any deviation from consistent technique is immediately apparent. The pitch range and physical dimension of the full size violin is however far more "manageable" than that of the guitar with say 15 frets and six strings. There are simply many more variations of note and scale positions compared with those of the violin! Classical plucking technique with guitar is (in my experience) one of THE MOST challenging motor skills to perfect. One is required to perfect EXTREMELY complex independent right hand finger movements ( thumb, index, middle - and some performers include ring and little finger too)- while simultaneously maintaining fluidic, precise placement of the left hand fingers (index, middle, ring, little). In essence, one multi- tasks very complex motor skills at a very rapid tempo - using both dominant and non dominant hands! Combined with the need to develop motor memory for the position of every possible note position on the fretboard makes mastery of the classical guitar a skill at least AS "difficult" (if not more so) than that of classical violin.
Here I DO NOT refer to chord progressions as played on guitar. These techniques are fairly easy to master. I note however that chorded music scores are NOT the norm for violin - the arched design of the neck makes bowing chorded structures very limited in practice!
All said, both violin and guitar are technically very "difficult" instruments -when played according to classsical styles. The rewards these offer however, are enormous! I can only but recommend that one surrenders to both!