In the anime we see Gotenks hit SSJ, SSJ2 (Very briefly), and then SSJ3. In my own opinion, the only reason Gotenks can utilize SSJ3 is because the compound nature of the fusion being able to handle the transformation for the duration it has been shown to be able to withstand. Mere minutes if my memory serves.
Non-canon source, but in my own work there is a portion of the story where Shihori, 13 years old at the time, transforms into SSJ3 through sheer rage. It puts such stress on her small and young body that it lasts no more than a minute at best. It leaves her completely at the mercy of her opponent.
So realistically where we see the only individual character, being Goku, use the transformation in a normal mortal body. It lasts what, 10 minutes and that's being generous. Until he was able to improve his physique even further and somewhat handle the stress of the ki drain on his body. A full grown, highly fit, adult male could barely handle the transformation. Until I see proof it could be mastered then it's a worthless transformation with more downsides than positives. It's like firing all the ammo you have in one giant salvo and not preparing for the contingency that it might turn into siege warfare.