All the truly GREAT folk legends played Guitar as first choice as accompaniment.

Bob Dylan
Donovan
Woody Guthrie
Phil Ochs
Roy Harper
Richard Thompson
Roger McGuinn.

The cittern, Luet, Bazouki etc are inferior curiosities that floundered in the survival of the fittest.

Piano is probably one of the better accompaniment instruments but not suitable for the folky masses becausr of it's size.