Usually if you touch a nerve you just feel a little twinge that causes something to flex briefly, however even if unlikely there is always a chance you can sever something serious and cause local paralysis, this is why the common inject spots are ones not very dense with nerves and arteries. For this reason though I always inject really slow and never jam it in there, that way if you are coming up on a nerve there is a good chance you put pressure and cause it to fire before slicing through it. All in all though I may have triggered one maybe once or twice, really not a big big deal.
Some people find the quadricep kind of painful, but I don’t mind, some people like some spots better than others. I put 2+ cc in my delt one time and it was fucking excruciating. In my ignorance I was icing the spot religiously too and in all likelihood dropping the solute out of suspension and making it worse. However I’m the same way in that I feel the need to have a steady hand, I cant even reach my delts in a way that I feel comfortable with.
Best spot ever though, ventral glute hands down. I want to say that the actual muscle you pierce through is the Tensor Fascia Latae (TFL). You can easily sit down, find it and reach it both hands from the front; it sort of pops out from the crease of your hip when you sit down. The least painful spot I have ever used. In fact I only use VG and outer quad, even on ED injections.
You can even inject most oils subQ with an insulin needle rather painless, however this is not practical for cycles involving much of fluid volume, as there is less blood flow and slower absorption. There is also a chance of accumulating oil in between the skin/fat layer and the muscle which will cause a knot that may take some time to go away. I did a short test prop and nandrolone prop cycle injecting subq ED and it was fine. I now however take a small weekly test dose of test E 400 subq @ .1ml ED and it causes swelling in the area for a handful of days, even at .05ml per site! .3 ml per site made me look like I implanted a golf ball under my skin.