"The Nikon Coolpix P510 has a massive 42x optical zoom: the lens stretches from 24mm to an astounding 1000mm equivalent. It's the first camera to break the 40x mark and the first to reach a four-figure focal length."
What are the risks and benefits of this camera compared to my D5100?
Smaller sensor, much poorer high-ISO performance. The smaller senosr makes it much harder to get subject isolation (i.e., nice out-of-focus effect behind yor subject - aka bokeh). At 1000mm equivalent, you'd need to be at least at 1/1000th shutter speed handheld or on a really good tripod. Performance likely not optimized at wide-open apertures (certainly over a large zoom range like that), so expect to be stopped down. Sharpness and contrast are the first image aspects to suffer at super telephoto (anything > 400mm) so it remains to be seen how good the lens really is. At 500mm, you're fighting atmospheric effects like haze between yourself and the subject.
Advatanges - small package, amazing zoom range in a single camera, relative convenience.