I'm surprised that the Bristol Belvedere hasn't been mentioned up until now.
A bit before my time, but I have talked with those who flew it, and feared it. The engines were started with Avpin (an explosive, and deadly corrosive liquid), and start fires were common.
They told me that they would not strap in until the engines were started. The concept was to stand in the jump seat position, lean forward to press the engine start switch, and be prepared to run!
In addition, the sync shaft between the two rotors was none too sturdy, and a failure of that in flight was fatal.