I'm not sure 'underestimated' is the word.
So, yes, I knew that babies wake up in the night and that you can't have lie-ins to catch up, etc., but, until you experience that, you can't know the impact it will have on you.
Of course you have no idea how your baby is going to be.
It was only after I had my 2nd, that I realised not all babies were awake for 2 - 3 hours ever time they woke up in the night, for example.
Also, quite frankly, if you want to have a family / have dc, then, even if you were aware you would really struggle with {insert pregnancy or sleeplessness, or breastfeeding, or the unending monotony, or whatever got to you the most}, you still would make a decision to 'get through' that, in order to get to the nice bits of parenthood.
In the same way few people enjoy studying and exams, but you know you need to get through them, to be able to do the job you enjoy.
Or you need to practice your sport in the freezing rain if you want to reach the heights of medals and top performances
Same wit practising scales and the boring bits of learning an instrument if you want to be able ti sit, as an adult, and play the instrument for pleasure
etc
etc
etc