The single biggest factor besides correct technique of the hand propper, is a good set of magnetos and spark plugs. So often I have seen people struggle with a hand prop because of weak spark which results in struggles with fuel, which then results in unintended RPMs and a resulting runaway. J-3s with A-C mags seem especially susceptible because so few are correctly maintained because of a paucity of skilled maintenance people on those units.
I used to be very good at hand propping even bringing some 6 bangers to life. No more. I am not current and no longer proficient.
I took quite a beating a while back on SuperCub.org because I challenged the conventional wisdom there that everyone should be able to hand prop their airplane. There is not reason to do this if a starter is available for several reasons. One, if it is starter equipped it is unlikely the person involved is proficient and current, both required for a safe start. As I pointed out, a dead battery for starter is not an excuse in the world of overnight freight. Some Alaska types said, "What if I am on a glacier!" Answer: Have someone fly you up a battery or starter. Hand propping when you are not current or proficient on an snow or ice covered surface is a really bad idea. Bottom line, like texting while driving...it can wait.
A while back at KUCY I saw a 172 after several failed start attempts ending with a dead battery attempt a hand prop. The technique was so bad including leaning into the swing, I finally had to walk over and suggest they stop before someone got killed. I rarely do something like that, but I decided it was so bad that if the prop caught, there would be blood.