Good video link provided. I appreciate Stumpy sharing this mishap. However, the message was flawed.
The cause of this accident was the fact that Stumpy erroneously worked the blade down into the bottom area of the dish of the seat. Once he allowed the blade to collide into an adjacent material plane (at a near right angle) the breaking action on the rotational thrust, exacerbated by the aggressive speed and power of the motor, was well beyond the capacity of his grip (simple physics) to overcome. Grinder then became the flesh eating projectile as described.
This condition can happen with erroneously working a chisel down into a bowl on a lathe. There are wood turning videos covering this type of danger and are well worth reviewing. Very dangerous kickback condition that, obviously, should be avoided. Sorry folks, it's not the tools fault in this case. It was user error. Maybe Stumpy could blame it on the tool if it failed (like a links broke and the disc flew off and into his body). But it didn't. He simply failed to maintain control over a powerful tool.
My suggestion is for him to make a follow up video and show the accident portion of the video in slow motion so that users (including STumpy) can be aware of exactly what happened...and why. That would make the video a learning experience rather than just another woodworking horror story. Hate to say it, but it's not fair to characterize tool design as inherently unsafe when user error is root cause of an accident. Another example of this flawed mentality is demonizing all pit bulls as inherently viscous attack animals. Sure they can be made to act that way if you abuse their nature--same as was done with this tool. Like any tool, the rotary chainsaw grinder blade has its place in woodworking utility and when used correctly, works phenomenally. I have one and have no intention of throwing it in the trash as this video suggests. Table saws are also very dangerous and accidents happen on them every day. Mainly because most folks do not appreciate the power involved or understand the nature of wood that is brought to the machine. Does that mean we should throw them in the metal scrap bin or all buy Sawstop technology? In my opinion both are absurd propositions.
I will add here that when I use this type of chain saw rotary blade I use it on a variable speed FEIN angle grinder. It an expensive grinder. But gives much more control and an additional factor of safety when using this tool with different discs and with different materials. Most folks buy the cheapest grinder out there which is almost invariably an on/off proposition-- at full bore. They expect these pieces of junk to be the "do all" for every rotational grinding situation. Big mistake. Stumpy made it as soon as he loaded that blade on his piece of junk and also failed to change his angle of tool attack when confronted with a change in material profile.
Woodworking is a rewarding, but dangerous occupation. Be safe every chance you take. Put the odds in your favor by truly learn from the mistakes of others every chance you can get.