The problem is that these designer hormones, like all steroid hormones, alter gene expression in the nuclei of the cells. A hormone will increase expression of some genes and reduce expression of others. It is like installing new drivers on your computer, some can make the computer unstable.
These designer [fake] hormones make some unintended changes to gene expression. With deca, guys can loose the ability to get erections. The root of that change is very likely in the brain. - how much fun is that? The classic example is how progestins cause women to have more cancers, blood clots, heart attacks and strokes. We see that a lot of guys get damaged by internet garbage 'legal steroids'.
Do not think that these designer hormones are testosterone, they are not and there is no understanding of what goes wrong and why so severe for some guys. The functioning of some cells can be broken.
As for aromatization, one can be on TRT and manage/optimize E2 levels very easily with anastrozole. So no need to be looking for a testosterone like drug that does not aromatize.
If you take a fake testosterone that cannot aromatize at all, then cells cannot make E2. E2 is very important for basic functioning of men and women. Remember that men have the female cellular mechanics, with small changes. If the brain cannot make E2 for local requirements, you have mental problems. [The adrenal glands still make some background T and E2.