Flaps 40 in a Twin Otter requires piloting skills just a little beyond the standard Caravan/Navajo/King Air. It's easy to get too much weight on the nose wheel, or even wheelbarrow it, which I have seen done. The nose wheel area (known as "station 60") is a little vulnerable to hard landings on the nose wheel. I was always afraid to be responsible for damage in general, and station 60 in particular - so I kept the nose light.
I was trained by a DHC test pilot, who would not let me fly 40 flap landings, until I really impressed him with my flaps 20 landings. He also trained me in 30 flap takeoffs, but that's a story for a different time...
The Twin Otter is a very sure plane to fly, so one feels pretty confident handling them - occasionally too confident, perhaps...