A short post. Niholas, with a confident rider and in the right situations, are surprisingly easy to drive very quickly downhill. Well in this case "very" means perhaps 35-40km/hr, to speeds where the wind roars in the ears, and guys in stretch pants riding racing bikes can't pass a coasting trike without pedaling. It basically gets easier the more weight is in the box, and the lower and more behind-the-front-axle that weight is. A loaded Nihola goes down a hill like a cannon ball. A 20kg kid or two really stabilizes things, and the trike runs smoothly through the corners and pavement imperfections, but an empty Nihola with well-pumped tires is a bit jittery. I've pedaled (sprinted) to about 35km/hr (based on a GPS track) but generally at those speeds its best to concentrate on smooth driving, and plan well ahead in case it might be necessary to stop. Definitely a good idea to remember that stopping takes a while.
Gravel downhills work great too, so long as the bumps are not large enough to start throwing people and trikes around. The tires and frame can smooth out a half-decent gravel road very well with a kid or two in the box, and it just seems to get smoother with speed... just avoid washboard and holes.
Niholas are also great fun on downhills when there are slippery corners. In this past winter I was doing three-wheel slides in the snow, and in the summer I have a time or two had the chance to push it to the limit on dirty pavement. I can't recall going nuts on gravel but I think it could be fun too. I've noticed that in the summer, when riding fast and using the back brake hard, it tends to have abrupt consequences on directional stability. Its perhaps more fun to squeeze 100% out of the front brakes and corner until the inside tire starts sliding.
Oh, and my kids love speedbumps.