Here's my take on it, having played 10- and 8-string guitars for some 35 years:
A major failing is the lack of "standardized tuning" for the added basses. Focusing just on 8-string options:
7=D, 8=C ("Romantic Era", good for Coste, etc)
7=D, 8=B (ren. lute)
7=B, 8=D (inverted ren. lute, and the one I prefer)
7=C, 8=D (inverted "Romantic Era")
8=A, 7 to 2 = normal guitar, 1= high A (Egberto Gismonti, Paul Galbraith)
(And the tuning variants available in the 10-string world get even MORE confusing!)
Secondary to this, I've often encountered the "uninformed audience" scenario when playing 10- or 8-string. This is most commonly observed at non-formal gigs, receptions, etc. "Wow, didja see the cool guitar this dude has? I'm waitin' for him to rip some fast scales on them top strings!!" (visually on top, but meaning the lowest basses), followed by an air-guitar demo with Van Halen sound effects. Usually within 30 more seconds, the same person says "Schitt, he ain't gonna do it!", gives a look of disgust, and walks away.
Salt Lake City, UT
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