This subject pops up from time to time.
The fact is that, at least in the UK, a significant number of flyers hand start their engines week in, week out, without problems. I am not aware of a single serious or fatal injury caused specifically by hand-starting in the last few decades but there have, I think, been a couple of runaway incidents. (And there have been unrelated propeller incidents. Propellers are dangerous!)
Owners are often told to have another "competent person" at the controls but most owners prefer to do it themselves. They have a routine and someone else involved destroys this. The owner gets to know his aircraft and hand-starting is a bit of a fine art particular to each aircraft.
If hand-starting an aircraft not known or not of a type that is normally hand-started (e.g. due to a flat battery) I would recommend at least two "competent persons" because the "fine art" model doesn't apply and one is in virgin territory. This is a very different scenario.