Can't believe nobody's mentioned STRESS. There's always something to worry about. Them getting older just means it's a different something.
Sleep, money, time, freedom, sex, time for your relationship, not damaging your body, not having to go to soft play centres and kids' parties and all the other horrible horrible things. Time for reading and hobbies and friends.
If you need more, just spend a bit of time looking at the stuff people post on mn. My toddler hits other kids, my 8yo eats nothing but crisps, my teen's on drugs and smashes windows when we say no to him. My 35yo throws a fit if I won't give her free childcare five days a week but otherwise doesn't want to know me. The things that can go really horribly wrong are infinite.
They're a nightmare, then they leave. Then you're sort of back to where you started, really, in terms of your day to day life. (Only with youth, energy, potential , options etc gone, obviously). People who don't do it are just skipping the twenty years of hard labour in between.