The entire series is a slow burn of nightmare fuel. Imagine having your wounded, dying body frozen, and then repaired with surgery, adding things in such as additional structures to your muscles and bones and such. You wake up during the process several times because you're not quite dead yet, and the whole process is keeping you alive.
They're still operating on you when this is happening. Somewhere down the line, you die, but they're not done with you. They reanimate and reheat your body through pituitary and thyroid augmentations. You come back to life, dazed, confused, terrified. After that, they give you your first injection, which stabilizes you, cutting you off from memories of your past, and leaving you feeling emotionally and mentally stable.
What you don't realize that the drug is violently suppressing who you are, leaving you numb, cold and ultimately obedient. Over the course of the next decade or so, they put you through training that would break even Tier One operators from the military and CIA, and outright kill anyone else below Special Forces. When you get broken, they fix you right up, or they stick you into a cold environment and let your body repair. Sometimes during these moments of recovery, the memory clearance wears off and you're aware that you're horribly injured and that your body is recovering at an abnormal rate. You panic, and they strap you in and inject you with more memory clearance cocktail. They repeat this process over and over, leaving you more empty and hollow, but with amazing mental clarity.
After it all, the surgery, the training and the high-concentrate nutritional supplement literally makes you a superhuman asskicker; a veritable supersoldier, but you lack free will and a conscience. "Totally programmable and erasable." In the long-term, these augmentations and such, leave you with an extremely high metabolism and core temperature, which they have to regulate with the nutritional supplement, because feeding your augmented ass would be incredibly expensive; and because not doing so, would leave you useless and unable to perform a task, or render you into a violent berserker, who treats unfamiliar faces as threats to pulverize into shit.
Another downside is that, without the memory clearance drug, you're prone to experience traumatic memories of what you've done along with memories of your past, in full, sharp, painful detail. And it doesn't just stop at flashbacks while you're awake. You have nightmares that would make H. R. Giger shit his internal organs out. To top off the overall nightmare of the procedure that makes you into a hardcase black ops cold warrior, once you're taken off the drugs that help make you superhuman and push your performance to those levels, you come out of it as the worst junkies seen by man.
The years of strain and performance takes its toll on you. You feel like you're thousands of years old. You're constantly feeling weak, confused and helpless. To make matters worse, you have to undergo long-term hormonal treatment, to return you to being remotely normal, and the process isn't exactly something most people would like to subject themselves to. The drugs and processes that also kept you in peak fighting condition, also slowed down your aging tremendously, but since those things are no longer circulating in heavy amounts in your body, you are now subject to aging and all the toils and woes associated with it. And that's just the tip of the frozen iceberg of nightmare fuel that comes with being a Universal Soldier.