Caprice number 39 is such a sweet, gentle piece - it was one that took no effort to practice, even though it's in F# major. Perhaps that was the point: dress up an unagreeable key in a beautiful melody!
Caprice no. 16 was a bear for my left hand, which I contorted in ways I previously thought impossible. I disliked it for a long time because frankly, I couldn't make it sound good. The very first chord in particular, with the extended fourth finger, is just barely playable for me on a 16.5-inch viola. However, after clearing the hurdles, I've decided it's one of my favorite caprices. The key of E Major sounds mellow, rich, and joyful. especially on the return of the "A" section after the C-sharp minor section (mm.17-end) . Campagnoli certainly does interesting things with the bariolage stroke, especially in mm. 9-16 where he staggers the slurs. This makes it extra tricky for the left hand, as if any more challenges were needed in a piece like this! I found it most helpful to emphasize the bottom note of each chord, to keep the line going and also to keep my bow and left hand on the from getting distracted by other technical complications.