Its definately having frets that make ALL the difference ! I 'busk along' with a number of singers and musicians at The Tap , in all sorts of keys , mostly on mandolin and find that WITH practice , the 'silly' keys are a lot easier , though if I didnt have frets it would be a HELL of a lot harder !
One Fiddler I Gig with (Five years at Guildhall School of Music) is QUITE happy playing tunes in ALL the keys , BUT he is a Smart Arse !!