Extreme dense packing may increase the chance of necrosis, but the more likely outcome is that follicles will be fighting for the same (lowered) blood supply in your dht sensitive scalp areas, thus sort of risking their long term health.
I've seen on numerous forums guys lose their transplanted hairs over time, but I'm not sure if that's due to the follicles getting androgen sensitive again, or because there isn't enough blood flow to nourish them properly.
It is certainly an area that I would like to know more about, and one the hair transplant industry doesn't talk about enough (for good reason, if it were found to be true with conclusive evidence and studies, this would certainly lower their revenue).