I'm an old pro at multi-pins/day, and never had any problems or reactions, other than everything being a little sore at the same time. Besides gear, I have a medical issue which requires twice/week deep IM injections. The only thing I had problems with, was when I tried enhancing oils to fix an imbalance on my upper two abs. IM injections into the abs hurts like a MOFO! I was doing 1cc daily on right & left upper two ab heads, and I had to use a 22g needle due to the thick oil. I did it for about 2 months. I have to say that it DID WORK, and in fact now a year later I think it's still more balanced looking than before I did it.
As far as injection spots, I've done them all, including ventroglute. Basically it's on the side of the hip, and just below the rounded end of the iliac crest (bone at the top of the butt which curves around to the front). I aim the needle slightly to the rear and slightly down, and go in about 1" or deeper. It's not my favorite spot to inject, but it does supposedly keep you less likely to hit the sciatic nerve, if I recall reading. I like the rear heads of my delts the best, and never have problems. I have really big delts (as compared to other bros my overall size), so I use a 1 1/2" needle and it's still clear of the bone and joint capsule. Pecs hurt a lot, and so do calves. If I've used up all the spots on the delts, then my #2 choice is quads or glutes. ****I'm NOT telling anyone that this is correct, and I'm NOT a doctor and NOT qualified to give any such advice!**** Just sharing personal experience as it affects ME.
Charles