I’d rather use a keyboard. I don’t think think that 8 string has any practical use because their downsides overwhelm the upsides.
Some downsides that come to mind:
=the extra complication of the added strings… Adding 2 strings doesn’t make it twice as difficult for the player as adding 1 string, it’s more like 4 or 5 times more difficult because of the number of relationships between the notes (look up factorials in maths).
=the extra weight
=the width of the neck, not to mention it’s past the point of no return in neckdive land.
=issues with the F# string! I have enough problems with the low B string.
=the extra cost getting all elements of the bass to work well together. Not everyone has the bass setup skill to make the best use of the low F#.
=it will most likely sound awful
=are their any cabs that can reproduce the sound accurately?
=price of strings.
So overall it’s one big fat juicy NOPE.