In my humble but probably accurate opinion, the mandolin
banjo (or banjo mandolin) is almost totally ignored today
because musicians quickly discovered that it combined
the worst, instead of the best, qualities of its parents,
i.e. the slender treble tone of the mandolin and the essential
'plunkiness' of the banjo. Missing are the sustain of the
mandolin and the volume of the banjo.
I speak as the proud owner of several of these beasts when
I say that they make lovely conversation-inspiring wall hangings
but serve no other purpose, musical or otherwise.
BTW I have a couple of eight-string tenor banjos (19-fret neck,
full-size pots) that I enjoy playing which in fact combine the
good qualities of the banjo and mandolin. Anyone interested in
knowing more about these yokes please contact me off-list.