Mandolin.
I'm a reasonably competent guitarist (40 years and counting) and over the years I've picked a number of stringed things and been quickly able to get an acceptable sound from them.
The problem is my fingers!
I've got really really thick fingers!!
My fingers are so thick that a guy once approached me after a gig and said..
"I just wanted to tell you, that after watching you over the years, you're the reason I decided to learn to play the guitar"
My flattered ego was quickly pricked by:-
"I thought that if you can play guitar with those fingers, then anyone can!"
That's why I don't play the mandolin. Because it's physically impossible!

C-flat (aka fat-fingers)