Genetics plays an important rule but it isn't the major factor here,
no body in the whole world can use 1000mg of Steroids without getting any side effects
side effects varies whether short term or long term side effects as an example I would use 1000mg of Test only and get acne, gyno etc.
or I wouldn't get any but this Steroid had increased the risks of getting prostate cancer not to get it in this year but maybe after 10years maybe more maybe less
another example, you used D-bol 40mg/day for 8weeks, it caused a lot of toxications and you didn't notice any side effects which is very common
then now your liver is very stressed and tumors can build-up now and appears after several years and not immediatly
infertility, let's say you used Steroids but it inhibited your Gonadotrophins production especially LH/FSH then the production of sperms will be very hard
maybe you can get a kid after 1month from the cycle as Testosterone matured Sperms but if you didn't recover very well, you can't get a kid if you're off for 3months or so untill you completely recover
heart diseases as elavation of Cholesterol or heart muscle enlagment which happen for many users and can kill you and symptoms cannot appear immediatly
Leukemia or blood cancer whether acute or chronic this type of cancer is related with Steroids usage especially Anadrol, D-bol and otehr Steroids which are responsible for increasing the production or red blood cells
Brain cancer especially with the usage of Growth hormone, do not appear immediatly needs at least 5 or even 20years as happend with the football star Lyle Alzado (GOD BLESS HIS SOUL)
and plenty of side effects needs a lot of years to appear
All Steroids will cause side effects it's much like smoking but we all are doing it