It's a bit off topic here, but I will answer because I don't want to start another thread right now.
Multi-scale basses present a way to deal with the "floppy B-string"problem. In order to get decent string tension on an electric bass with a low B string, there are basically two options available - you can use a heavier gauge string or increase the string length, both of which will require a higher tension to bring the string to pitch. There are compromises with both solutions.
If you increase the string gauge, you increase the string's mass per unit length. The compromise here is that as the mass per unit length increases so do the unmusical overtones, and the note definition diminishes. And increaseing the gauge of all the strings will result in a lot of tension on the G string and even the D. But if you use just a heavier gauge on the B and E strings and leave the upper strings alone, the tonal difference between the lower and higher strings becomes more pronounced.
If you increase the string length you also increase your stretch to reach the lowest notes. And you increase the string tension on all of the strings simultaneously, likely giving undue tension to the D and G strings.
So the compromise is a fanned fret system which gives a longer scale to the low strings and a shorter one to the high strings. You can get a better definition on the B string than by using a heavier gauge, and you get more even tonal quality across all strings. Sounds Ideal - except for that "extra reach" problem.
And that extra reach and the geometry of the fanned frets is something that I really dislike. First consider the natural position of the hand in a resting position - stress free:
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When holding an electric bass it looks like this:
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It is a relatively natural position when playing mid-neck - the arm and hand are in a relaxed position, the elbow just slightly closer to the body than a fully relaxed position:
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But when you reach for the first position your hand takes an unnatural position, you must angle your wrist and push your elbow forward to make the reach. And if the reach is 36" rather than 34" the position is more extreme. And when you are dealing with frets that are angled away from your natural hand position the problem is at its worst:
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Not onbly does the wrist have to be twisted and angled, but the elbow is pulled into the ribs while simultanously extending the arm. Try it and note the stress in you hand, wrist and elbow. You don't have this problem when playing double bass - the hand position is much closer to natural:
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After decades of playing the electric bass (regular 34" and 35" scales) I have developed tendonitis and arthritis in my left hand and elbow. These days I play far more double bass and am not finding the same stress levels.
My analysis is that the fanned system helps to alleviate some shortcomings of the electric bass while adding to the stress on the human body.
My opinion is that the advantages are outweighed by the risks.