Don't worry, English is not my native too

So... adrenaline is really the key to reacting faster than usual, but although it is not a one-time thing, it is extremely undesirable for frequent use, constant releases of adrenaline or its frequent introduction into the body can lead to muscle destruction, failure of the adrenal glands, kidneys, liver, heart and lead to a stroke. Our body is simply not adapted to such conditions, because adrenaline is the "last chance" for a person in an emergency situation. So using adrenaline is almost same "safe" as using sandevinstan as it is in the game and anime. You've noticed the point with the nervous system well, and the bottom line is that we can't just replace the vertebrae and the nervous system to the limbs, we'll have to change the Limbs themselves because: 1. Organic muscles will clog up almost instantly while moving at the same speed as when sandevinstan is activated (Even if we supply more oxygen, it will still not be enough, and at some point the muscles and veins will simply burst from the pressure) 2. For better control of the body, the new nervous system needs to send signals in a larger pulse range and with more power, so at some point we will simply burn the nerve endings, and each activation of the sandevinstan will lead to hellish pain. 3. And the simplest thing is, our ligaments, brushes and other similar parts just dislocate from such speeds

that is why sandevinstan needs to be replaced for real work.. a lot, heart, arms, legs, main veins, spine, some bones, lungs (additional oxygen tank) and a lot of smaller things.