I paddled the carbon Nelo as a once-off in Hout Bay in Cape Town last year on a 10 km race. Some proviso's- I am not an elite paddler...middle of the bunch. Conditions were choppy with the race comprising upwind, downwind and cross-wind sections. I paddled it only once and hadn't paddled it before the race.
My findings were that it was very well made and light. Although I didn't swim, I found it unstable and had to brace much more than usual, but this had a lot to do with the conditions. On flat water with no wind I was quite comfortable. The stability was very similar to a high end racing K1 and the seating position was similar. Certainly more unstable than a FENN Millenium, which I had paddled before and was always considered unstable by reputation, but was fine for me.
I paddled a FENN Elite straight afterwards and that felt exceedingly stable after the Nelo. My usual boat is a FENN Mako6.
Despite the stability issues I did about the same time as I usually do on the race, so it is fast and I might have got more used to it given more time in the bucket. It felt most comfortable on the short downwind section and accelerated and picked up runs beautifully.
For an Elite paddler or on flat water, it would be very good, but at my level I would need a more consistently stable boat.
Gary