If the coil just breaks, the engine will just keep on going (most of the times)... But the next start will be a bit of a problem... (it won't work)

If a part of the coil gets released (and passes through the exhaust port without doing further damage) the engine will keep on running at full throttle, but it will sag a little... If you go to part throttle or idle when this happens, it will die most of the times...

If the plug coil completely fails (releases) the engine will quit... and unless you are a very lucky person you will be shopping for a new liner and piston...

So use the right plug... and it won't fail... think about it...