I have used a "Unicorn" log splitter. Yes they are very fast and effective. Yes, if you screw up, you will get screwed up. They typically aren't sold anymore. Probably similar to running a chuck and duck chipper or a buzz saw. If you are careful, they get the job done. Inattention can lead to getting too wrapped up in your work. I sold it "cheap" and made sure the buyer was aware of its hazards. (they were specifically looking for one so I guess they knew what it was)
Any type of powered equipment has its hazards. I prefer a maul or my slow hydraulic splitter.