Honestly I have found that proper hygiene is the most important part of injection. As long as I am extremely careful to keep everything sterile, I can injection: lats, glutes, biceps, traps, and delts, zero pain minus the initial piercing.
I think the main reason I can’t pin quads is because I am always on my feet, and the muscle gets super irritated with the extra volume: a lot of nerves little space for any inflammation.
That said the difference between a good pin and weeks of agony is sterilization: clean drugs, clean pins, clean injection site.