These effects are experienced with the compound itself : Nandrolone.
The ester has absolutely nothing to do with anything.
Also making claims such as "Deca stays in your system for 18 months" is hogwash. The metabolites linger around in your system. The metabolites have nothing to do with the active drug itself. The metabolites of NPP linger around almost just as long.....
I can see why some would say that NPP would be better than DECA for health purposes, in the sense that NPP gets in and out quicker. However, I really highly doubt there is really any kind of difference "health wise" between the two esters.
If you take 600deca for 10-12 weeks, most of that time is spent simply building up the compound to actually deliver the amount you're really trying to get in your system. I would say you have far more time of having less MG per MG ester available in your system to do "damage" than if you took 600npp. 600npp will be in your system very quickly that entire 10-12 weeks, and would do "more damage" imo. This really boils down to the length of the cycles, I suppose.
Regardless, this is all speculation. I can somewhat understand the point that the 600deca will "linger in your system" for a few weeks after that 10-12 weeks - but so what? The NPP was in your system at a much higher dose in those beginning weeks(quite possibly up to about 8 weeks at a higher dose than 600deca). Either way, you can't say either is safer; I would argue in this scenario of comparing 600deca vs 600npp for 10-12 weeks , that the 600npp has a lot more time to "deal damage" at a higher mg. Could be wrong...but this still is not really the point of the subject.
Perhaps if we dragged it out to 12-16 weeks , then we could say "NPP would be better" because it's less time the drug is in your system and NPP does not have the chance to have the 'compound effect' that long esters have, which causes the MG of substance in your body to become even higher. Still, not the point.